Method and apparatus for transmitting/receiving synchronization signal in device-to-device communication system

ABSTRACT

A synchronization signal transmission/reception method for use in a device-to-device (D2D) communication system is provided. A method of transmitting a synchronization signal for D2D communication in an asynchronous network, in which a first terminal located in a first cell controls synchronization with a second terminal located outside of the first cell, includes transmitting, at the first cell, a system information block (SIB) including resource allocation information of the first cell and at least one neighboring cell, determining whether the first terminal is in a first mode and whether the first terminal is scheduled to perform one of a D2D communication and a D2D discovery; determining, when the first terminal is in the first mode, whether the first terminal is required to transmit a D2D synchronization signal (D2DSS); and transmitting, when the first terminal is required to transmit the D2DSS, scheduling information including a transmission time point and transmission reference timing information to the first terminal.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation application of prior application Ser.No. 14/818,573, filed on Aug. 5, 2015, which claimed the benefit under35 U.S.C. § 119(e) of a U.S. Provisional application filed on Aug. 6,2014 in the U.S. Patent and Trademark Office and assigned Ser. No.62/033,798, of a U.S. Provisional application filed on Sep. 25, 2014 inthe U.S. Patent and Trademark Office and assigned Ser. No. 62/055,232,and of a U.S. Provisional application filed on Nov. 6, 2014 in the U.S.Patent and Trademark Office and assigned Ser. No. 62/076,156, the entiredisclosure of each of which is hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to device-to-device (D2D) communication.More particularly, the present disclosure relates to a synchronizationsignal transmission/reception method for use in a D2D communicationsystem.

BACKGROUND

To meet the demand for wireless data traffic having increased sincedeployment of fourth generation (4G) communication systems, efforts havebeen made to develop an improved fifth generation (5G) or pre-5Gcommunication system. Therefore, the 5G or pre-5G communication systemis also called a ‘beyond 4G network’ or a ‘post long term evolution(LTE) system’.

The 5G communication system is considered to be implemented in higherfrequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higherdata rates. To decrease propagation loss of the radio waves and increasethe transmission distance, the beamforming, massive multiple-inputmultiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna,an analog beam forming, large scale antenna techniques are discussed in5G communication systems.

In addition, in 5G communication systems, development for system networkimprovement is under way based on advanced small cells, cloud radioaccess networks (RANs), ultra-dense networks, device-to-device (D2D)communication, wireless backhaul, moving network, cooperativecommunication, coordinated multi-points (CoMP), reception-endinterference cancellation and the like.

In the 5G system, hybrid frequency shift keying (FSK) and Feher'squadrature amplitude modulation (FQAM) and sliding window superpositioncoding (SWSC) as an advanced coding modulation (ACM), and filter bankmulti carrier (FBMC), non-orthogonal multiple access (NOMA), and sparsecode multiple access (SCMA) as an advanced access technology have beendeveloped.

With the popularization of smartphones, data traffic is increasingrapidly. The increasing number of smartphone users spurs the consumptionof smartphone-based application services such as social network services(SNSs) and games, resulting in an unprecedented increase in datatraffic. Of particular concern, beyond person-to-person communication,if machine intelligence-based communications, such as person-to-machineand machine-to-machine communications, are activated as a new field oftechnology, traffic concentrations at base stations will increase toodramatically to accommodate the increased demand.

There is therefore a need of a technology to accommodate increasedtraffic, and direct communication between devices is coming into thespotlight as such a technology. This technology, so called D2Dcommunication, is promising for licensed band communication systems suchas cellular communication systems and unlicensed band communicationsystems such as wireless local area network (WLAN) systems.

LTE-based D2D communication is characterized by D2D discovery and D2Dcommunication. D2D discovery is a process in which a user equipment (UE)checks identities or interests of other proximally located UEs, oradvertises its identity or interests to other proximally located UEs. Atthis time, the identity and interests may be represented by a UEidentifier (ID), an application identifier, or a service identifier.Further, the identities and interests can be variously configureddepending on the D2D service and operation scenario.

In order to support D2D operation between D2D, UEs located in differentcells in an asynchronous network, it is necessary to perform asynchronization procedure between the Tx and Rx D2D UEs. For thispurpose, the Tx D2D UEs located in the respective cells transmit a D2Dsynchronization signal (D2DSS). There is therefore a need to implementrules for determining the D2DSS transmission timing, D2DSS Tx UE, andresources for D2DSS transmission. In a case where a plurality of UEstransmit D2DSS, an Rx UE may receive diverse D2DSSs. Accordingly, it isnecessary to determine the timing and resources for transmitting eachD2DSS. In a case of the LTE-based D2D system, this determination must bein compliance with the resource allocation and transmission rules of theLTE system.

However, no specification on D2DSS transmission timing and resourceutilization of the D2D Tx UE has yet been proposed. Furthermore there isa need of specifying the operation of the D2D Rx UE in association witha Type 2B discovery transmission timing and a Mode 2 communicationtransmission timing configuration. However, there is currently no methodspecified for implementing such mechanisms.

There is therefore a need to specify resource allocation forsynchronization between transmitter and receiver and operation of thereceiver according to the determined transmission time and transmissiontiming in the system supporting D2D communication.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present disclosure.

SUMMARY

Aspects of the present disclosure are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentdisclosure is to provide a method and apparatus for allocating resourcesfor synchronization between a transmitter and a receiver in a systemsupporting device-to-device (D2D) communication.

Another aspect of the present disclosure is to provide an apparatus andmethod for facilitating data transmission/reception using a signaltransmission time and resources of a transmitter in a system supportingD2D communication.

Another aspect of the present disclosure is to provide a receiveradapted to the synchronization signal resource allocation scheme,transmission time, and transmission timing, and to provide a method forcontrolling the operations of the receiver.

In accordance with an aspect of the present disclosure, a method oftransmitting a synchronization signal for D2D communication in anasynchronous network in which a first terminal, located in a first cell,controls synchronization with a second terminal, located outside of thefirst cell, is provided. The method includes transmitting, at the firstcell, a system information block (SIB) including resource allocationinformation of the first cell and at least one neighboring cell,determining whether the first terminal is in a first mode and whetherthe first terminal is scheduled to perform one of a D2D communicationand a D2D discovery, determining, when the first terminal is in thefirst mode, whether the first terminal is required to transmit a D2Dsynchronization signal (D2DSS), and transmitting, when the firstterminal is required to transmit the D2DSS, scheduling informationincluding a transmission time point, and transmission reference timinginformation to the first terminal.

In accordance with another aspect of the present disclosure, a method oftransmitting a synchronization signal for D2D communication in anasynchronous network in which a first terminal located in a first cellestablishes synchronization with a second terminal located outside ofthe first cell is provided. The method includes receiving, at the firstterminal, a SIB including resource allocation information of the firstcell from the first cell, determining whether the first terminal isoperating in a first mode for transmission as scheduled by the firstcell, transmitting, when the first terminal is operating in the firstmode, a D2DSS based on scheduling information received from the firstcell, determining, when the first terminal is operating in a second modefor performing at least one of communication and discovery by selectingcertain resources based on the resource information included in the SIB,whether a D2DSS trigger condition received from the first cell isfulfilled, and transmitting, when the D2DSS trigger condition isfulfilled, the D2DSS, wherein the D2DSS is transmitted according todownlink transmission timing in the second mode.

In accordance with still another aspect of the present disclosure, amethod of receiving a synchronization signal for D2D communication in anasynchronous network in which a first terminal located in a first cellestablishes synchronization with a second terminal located outside ofthe first cell is provided. The method includes receiving, at the secondterminal, a SIB including resource allocation information of the firstand second cells from a second cell serving the second terminal,receiving a D2DSS from the first terminal based on the D2D resourceallocation information of the first cell, receiving a maximum timingadvance (TA) value of the first cell through a physical D2Dsynchronization channel (PD2DSCH) in a subframe carrying the D2DSS,determining a slot boundary based on the received D2DSS, configuring areception window for one of a D2D discovery message and a D2Dcommunication signal based on the slot boundary and the maximum TAvalue, and receiving at least one of the D2D discovery and the D2Dcommunication signal from the first terminal during the receptionwindow.

Other aspects, advantages, and salient features of the disclosure willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses various embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainembodiments of the present disclosure will be more apparent from thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1A is a diagram illustrating resource allocation for an inter-celldevice-to-device (D2D) discovery in a long-term evolution (LTE) D2Dsystem according to an embodiment of the present disclosure;

FIG. 1B is a diagram illustrating a bitmap representing a resource poolin an intra-cell D2D discovery according to an embodiment of the presentdisclosure;

FIGS. 2A and 2B are diagrams illustrating a resource allocation forintra-cell D2D communication in an LTE D2D system according to variousembodiments of the present disclosure;

FIGS. 3A and 3B are diagrams illustrating a scheduling assignment (SA)and data resource allocations for D2D communication according to variousembodiments of the present disclosure;

FIGS. 4A and 4B are diagrams illustrating an SA and a data resourceallocation in a case where Mode 1 and Mode 2 SAs are frequency divisionmultiplexed (FDM'ed) according to various embodiments of the presentdisclosure;

FIG. 5 is a diagram illustrating a resource allocation for a timedivision multiplex (TDM) of an SA and data according to an embodiment ofthe present disclosure;

FIG. 6 is a diagram illustrating resource allocation for an inter-cellD2D discovery according to an embodiment of the present disclosure;

FIGS. 7A, 7B, and 7C are diagrams illustrating a D2D synchronizationsignal (D2DSS) Tx resource allocation for an inter-D2D communicationaccording to various embodiments of the present disclosure;

FIG. 8 is a conceptual diagram for explaining a D2D Tx user equipment(UE) selection operation according to an embodiment of the presentdisclosure;

FIG. 9A is a timing diagram for explaining inter-symbolinterference/inter-carrier interference (ISI/ICI) occurring when a UE-Atransmits a Type 2B discovery according to uplink (UL) reference timingafter transmitting a D2DSS according to downlink (DL) reference timingaccording to an embodiment of the present disclosure;

FIG. 9B is a timing diagram for explaining an ISI/ICI occurring when aUE-B transmits a Type 2B discovery according to UL reference timingafter a UE-A transmits a D2DSS according to DL reference timing,according to an embodiment of the present disclosure;

FIG. 10 is a diagram for explaining a problem occurring when aneighboring cell Rx UE receives a D2DSS in a Type 2B discovery and thenreceives a Type 2B discovery message transmitted in compliance with ULtiming according to an embodiment of the present disclosure;

FIG. 11 is a block diagram illustrating a configuration of an e-Node B(eNB) according to an embodiment of the present disclosure

FIG. 12 is a block diagram illustrating a configuration of a D2Dcommunication-enabled UE according to an embodiment of the presentdisclosure;

FIG. 13 is a flowchart illustrating a control procedure of an eNB forD2DSS transmission of a UE according to an embodiment of the presentdisclosure;

FIG. 14 is a flowchart illustrating a D2DSS transmission procedure of aUE according to an embodiment of the present disclosure; and

FIG. 15 is a flowchart illustrating a signal reception procedure of a UEfor receiving a D2D discovery, a control signal or data from another UElocated in a neighboring cell according to an embodiment of the presentdisclosure.

Throughout the drawings, it should be noted that like reference numbersare used to depict the same or similar elements, features, andstructures.

DETAILED DESCRIPTION

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of variousembodiments of the present disclosure as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the various embodiments describedherein can be made without departing from the scope and spirit of thepresent disclosure. In addition, descriptions of well-known functionsand constructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of the presentdisclosure. Accordingly, it should be apparent to those skilled in theart that the following description of various embodiments of the presentdisclosure is provided for illustration purpose only and not for thepurpose of limiting the present disclosure as defined by the appendedclaims and their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

First, device-to-device (D2D) communication technology as a basis of thepresent disclosure is described.

It is assumed that protocol layers of D2D-enabled user equipment (UE)include a D2D application layer, a D2D management layer, and a D2Dtransport layer. The D2D application layer accommodates D2D serviceapplication programs running on the operating system (OS) of the UE, theD2D management layer is responsible for the function of converting thediscovery information generated by a D2D application program to a formatsuited to the transport layer, and the transport layer corresponds to aphysical/media access control (PHY/MAC) layer of a long-term evolution(LTE) or a wireless fidelity (Wi-Fi) wireless communication standard.

In the D2D communication technology, the D2D discovery is performed inthe following procedure. If the user executes the D2D applicationprogram, the application layer generates discovery information to theD2D management layer. The management layer converts the discoverinformation received from the application layer to a management layermessage. The management layer message is transmitted through thetransport layer of the UE. The UEs receiving the message operates in thereverse order of the transmission process.

The D2D communication is a communication method of exchanging trafficbetween UEs without assistance of any infrastructure entity such as ane-Node B (eNB) or an access point (AP). The D2D communication may beperformed based on the result of the D2D discovery procedure (i.e., withthe discovered UEs) or without D2D discovery procedure. Whether the D2Ddiscovery procedure is required before the D2D communication depends onthe D2D service and operation scenario.

The D2D service scenarios may be categorized into commercial service ornon-public safety service and public safety service. The services mayinclude a countless number of examples such as advertisement services,social network services (SNS), game services, and public safety service.

The aforementioned types of services are described briefly hereinafter.

First, a description is made of a case where the D2D technology isapplied to advertisement services.

A communication network operator supporting D2D allows preregisteredstores, cafes, movie theaters, and restaurants to advertise theiridentities to the D2D users located within a short distance using theD2D discovery or D2D communication. At this time, the interests mayinclude advertisers' promotion, event information, and discount coupon.If the corresponding identity match the interest of the user, the usermay pay a visit to the corresponding store to collect more informationthrough the legacy cellular communication or the D2D communication. Inanother example, a personal user may discover a taxi around him/herthrough the D2D discovery and exchange data about destination or farethrough the legacy cellular communication or D2D communication.

Second, a description is made of a case where the D2D technology isapplied to SNS.

The user may send other uses located within a short distance the user'sapplication and interests in the corresponding application. At thistime, the identity or interest used for D2D discovery may be a buddylist or the application identifier. The user may share contents such asphotos and videos with the neighboring users through the D2Dcommunication after the D2D discovery.

A description is made of a case where the D2D technology is applied togame services.

The user discovers other users and game applications for playing amobile game with the neighboring users through the D2D discoveryprocedure and perform D2D communication for transmitting data necessaryfor the game.

Finally, a description is made of a case where the D2D technology isapplied to public safety services.

The police and firefighters may use the D2D communication technology forthe public safety purpose. That is, in a case where the cellularcommunication is not available due to cellular network breakage causedby emergency situation such as fire and landslide or natural disastersuch as an earthquake, a volcano eruption, and a tsunami, such that thepolice and firefighters may discover neighboring colleagues or share theemergency situation information with neighboring users using the D2Dcommunication technology.

The current 3^(rd) generation partnership project (3GPP) LTE D2Dstandardization is directed to both the D2D discovery and D2Dcommunication but different in standardization range. The D2D discoveryaims to commercial use and thus has to be designed to operate in thenetwork coverage area of the eNB. That is, the D2D discovery is notsupported in the situation where no eNB exists (or out of the coveragearea of an eNB).

Meanwhile, the D2D communication aims to the public safety and disasternetwork service other than commercial use. Thus the D2D communicationhas to be designed to support both in and out of network coverage and ina partial network coverage area of the eNB (a communication in asituation where some UEs are located in the coverage area of the eNB andother UEs are located out of the coverage area of the eNB). Accordingly,the public safety and disaster network service is provided through theD2D communication without support of the D2D discovery.

In the LTE D2D of which the standardization is in progress, both the D2Ddiscovery and D2D communication are performed in LTE uplink subframes.That is, the D2D transmitter transmits D2D discovery signal and data forD2D communication in the uplink subframes, and the D2D receiver receivesthem in the uplink subframes. In the current LTE system, the UE receivesdata and control information from the eNB through downlink and transmitsdata and control information to the eNB through uplink, but theoperations of the current D2D transmitter/receiver may differ from thosein the legacy LTE.

For example, the D2D function-enabled UE has an orthogonal frequencydivision multiplexing (OFDM)-based receiver to receive the downlink dataand control information from the eNB and a single carrier-frequencydivision multiplexing (SC-FDM)-based transmitter to transmit uplink dataand control information to the eNB. However, the D2D UE has to supportboth the cellular mode and D2D mode. Thus, the D2D UE has to have anextra SC-FDM receiver to receive the D2D data and control information inuplink as well as the OFDM-based receiver to receiving downlink datafrom the eNB and the SC-FDM-based transmitter to transmit uplink data orcontrol information to the eNB.

The current LTE D2D defines two types of D2D discovery schemes accordingto resource allocation scheme.

(1) Type 1 discovery: The eNB broadcasts the uplink resource poolavailable for D2D discovery in a system information block (SIB) for allD2D UEs within the cell under its control. At this time, the resourcesize available for D2D (e.g., x consecutive subframes) and period ofresource (e.g., repeating at every y seconds) are informed. If thesending D2D UEs which have received the information select the resourcefor transmitting D2D discovery signal in a distributed manner. At thistime, the resource selection may be performed in various manners. Thesimplest method is to select the resource randomly. That is, the D2D TxUE may select the resource for D2D discovery randomly in Type 1 theresource region indicated from the SIB. Meanwhile, the Rx D2D UEsreceive and decode all D2D discovery signals transmitted in the resourcepool indicated by the SIB. The Type 1 discovery allows for the UEsoperating in a cellular radio resource control (RRC)_Idle mode and anRRC_Connected mode to transmit/receive D2D signals.

(2) Type 2 discovery: The eNB notifies the receiving D2D UEs of thediscovery resource pool using the SIB. The Tx discovery signal resourcesfor the sending D2D UEs are scheduled by the eNB. That is, the eNBcommands the D2D Tx UEs to transmit signals at predeterminedtime-frequency resources. At this time, the eNB may perform schedulingin a semi-persistent manner or a dynamic manner and, for this purpose,the D2D Tx UE has to transmit a scheduling request or D2D Tx requestincluding buffer status report (BSR) to the eNB. The D2D Tx UE also hasto operate in the cellular RRC_Connected mode to use the Type 2discovery. That is, the D2D Tx UE in the RRC_Idle mode has to transitionto the RRC_Connected mode through the random access procedure for D2D Txresource request.

Like the D2D discovery method, the D2D communication method also has twooperation modes determined according to resource allocation scheme.

(1) Mode 1: In a case of operating in Mode 1, the eNB or a release 10relay notification of the resource transmitting data and controlinformation for D2D communication of the D2D transmitter.

(2) Mode 2: In a case of operating in Mode 2, the D2D transmitterselects the resource in a distributed manner in the correspondingresource pool autonomously based on the resource pool informationacquired for data and control information transmission.

The aforementioned D2D discovery and D2D communication should besupported between the UEs located in different cells as well the UEslocated in the same cell. The present disclosure relates to a method forperforming D2D operations between the UEs location in different cells inthe cellular-based D2D system. Particularly, the present disclosurerelates to a method for a UE located in a cell of an asynchronousnetwork in which the eNBs are not synchronized to transmit a D2D signalto a UE located in a neighboring cell and a method for a UE located in acell to receive the D2D signal transmitted by another UE located in aneighboring cell.

A description is made of the operations of the eNB and UE in theIntra-cell D2D network.

(1) Synchronization

UEs receive the primary synchronization signal/secondary synchronizationsignal (PSS/SSS) transmitted by an eNB and perform downlinksynchronization with the eNB. Meanwhile, the UEs located in the cell fortransmitting uplink data and control information perform uplinksynchronization with the eNB. The uplink synchronization is establishedthrough a random access (RA) procedure, and the UEs receive timingadvance (TA) information from their serving eNB in the uplinksynchronization procedure. If the TA information is received, the UEstarts a TA timer and maintains the TA value received from the eNB untilthe timer expires. That is, if the TA information is received from theeNB, the UE uses the corresponding TA for transmitting uplink controlinformation and data until the TA timer expires. If the TA timerexpires, the RA procedure is performed again to acquire new TAinformation.

In the Type 1 discovery, the D2D discovery signal is transmitted incompliance with the downlink reference timing. That is, the D2Ddiscovery signal is transmitted in compliance with the time when thePSS/SSS is received from the eNB. Meanwhile, the reference time fortransmitting D2D discovery signal is not determined yet in the Type 2Bdiscovery. That is, in the Type 2B discovery, there is no specificationon whether to match the transmission timing of the D2D discovery signalwith the downlink reference timing like the Type 1 discovery or anuplink reference timing unlike the Type 1 discovery, i.e., TA-basedtransmission like legacy cellular terminal.

(2) Resource Allocation

The eNB may transmit the following information through an SIB forsupporting the intra-cell D2D operation.

Discovery type: The eNB transmit the information on the discovery typesupported in its cell (Type 1 or Type 2 or both). Similarly, theinformation on the communication mode (Mode 1 or Mode 2 or both) istransmitted for the D2D communication. Meanwhile, the eNB transmits theinformation on whether its cell support only the discovery service, onlythe communication service, or both the discovery and communicationservices.

Transmission pool: The transmission pool information can be applied toonly the Type 1 discovery and Mode 2 communication. All intra-cell D2DUEs receive the transmission pool information from their serving eNB.The transmission pool information may include the information on thestructure of the transmission pool. For example, it may include a numberof subframes forming the transmission pool and a number of resourceblocks (RBs) forming a subframe. Such information can be expressed invarious formats. For example, the D2D subframe configuration informationmay be expressed in the form of a bitmap, e.g., 1011100 . . . (1 for D2Dsubframe and 0 for cellular subframe). In a case of transmitting thesubframe configuration information in the form of the bitmap, each bitmay be set to 1 for D2D subframe or 0 for cellular subframe. Of course,it is obvious to those skilled in the art that each bit of the bitmapmay be set to 0 for D2D subframe or 1 for cellular subframe. The numberof RBs constituting the D2D subframe may be configured with the startand end points of the RB on the frequency axis. There may be morevarious methods for providing the Tx resource pool information.

The UE attempting to transmit a D2D discovery signal in Type 1 discoveryand the UE attempting to transmit data and control information in Mode 2communication may select the transmission resource in the transmissionpool indicated from the eNB.

Reception pool: The reception pool information can be applied to all ofType 1/Type 2 discovery and Mode 1/Mode 2 communication. All intra-cellD2D UEs receives the reception pool information from the eNB. Thereception pool information may include the information on the structureof the reception pool and configured in the same way as the transmissionpool information. All intra-cell D2D UEs receive and decode all RBsexisting in the discovery resource pool. Meanwhile, it is characterizedthat the transmission pool is a subset of the reception pool. That is,there may be a transmission pool consisted of N subframes in thereception pool consisted of M subframes. At this time, the relationshipbetween M and N may be M≥N.

A description is made of the inter-cell D2D operation in an asynchronousnetwork.

(1) Synchronization

In the synchronous network, the eNBs synchronize their transmission andreception timings one another using a global positioning system (GPS).Thus, in the synchronous network, all intra-network eNBs aresynchronized among each other. In the asynchronous network, however, theeNBs do not use GPS and thus not synchronized among each other. In acase where the eNBs are not synchronized, the inter-cell interferenceproblem is more significant in comparison to the synchronous network. Inorder to solve the synchronization problem, in the asynchronous network,the network synchronization is performed through X2 interface betweenthe eNBs or S1 interface between an eNB and a higher the level entity(e.g., mobility management element (MME)) of the Core Network. However,although the inter-eNB synchronization is performed using the networksynchronization protocol, it is difficult to secure subframe-levelsynchronization accuracy. That is, the neighboring cell B may have atime offset being earlier or later up to ½ subframe (0.5 ms) at thesubframe boundary of cell A.

In the Type 1 discovery or Mode 2 communication, the D2D UEs transmitD2D signals in compliance with the downlink reference timings of theirserving eNBs. Accordingly, there is a need of establishingsynchronization among the D2D UEs located in the different cells in theasynchronous network including non-synchronized eNBs not synchronized.In the LTE Release-12 D2D, it is determined that the X2 interfacebetween eNBs is not used for supporting inter-cell D2D operation. Thatis, the timing information of the neighboring cell is not provide to theUE served by the serving cell through the X2 interface. Accordingly, aD2DSS is defined to perform synchronization among the UEs located indifferent cells without timing information of the neighboring cell.

(2) Resource Allocation

In order to support D2D operation between D2D UEs located in differentcells of the synchronization network without timing information exchangebetween neighboring cells through the X2 interface, operationadministration maintenance-based (OAM-based) resource allocation isconsidered. That is, the higher level entity of the network, e.g., MME,acquires the timing information (system frame number (SFN)) of the eNBsunder its control through the S1 interface. Using the timinginformation, the eNBs can allocate the D2D resources. Particularly, itis characterized that the cells allocate the D2D resources not to beoverlapped on the time axis. Assuming that there are cells A, B, and C,the D2D resource pool of the cell A is allocated for the time period T1(corresponding M1 subframes), the D2D resource pool of the cell B forthe time period T2 (corresponding to M2 subframes), and the D2D resourcepool of the cell C for the time period T3 (corresponding to M3subframes). At this time, the time durations T1, T2, and T3 should notbe overlapped.

The D2D pool may be configured with consecutive D2D subframes ornon-consecutive D2D subframes. For example, if the D2D resource pool ofthe cell A is allocated with consecutive D2D subframes, the D2D resourcepool allocated during the time period T1 is composed of the D2Dsubframes (i.e., M1 subframes are all sued for D2D). Meanwhile, if theD2D resource pool of the cell A is allocated with non-consecutive D2Dsubframes, the subframes for cellular subframes and D2D subframes aretime division multiplexed (TDM'ed) in the time period T1.

The serving cell transmit the resource allocation information of theneighboring cell to the UEs using SIB. That is, the serving informs theUEs of the D2D resource pool of the neighboring cell as well as its own.The UEs perform D2D transmission and reception procedure using theresource allocation information of the serving and neighboring cellswhich is received from the serving cell. At this time, the D2Dtransmission may be performed in the serving cell D2D transmission pool(Type 1 discovery or Mode 2 communication) or on the specifictime-frequency resource in the serving cell D2D reception pool under thecontrol of the eNB (Type 2 discovery or Mode 1 communication).

Meanwhile, the D2D reception is performed in both the serving cell D2Dreception pool and neighboring D2D reception pool. For example, the TxD2D UEs of the cell A transmit signal using the D2D transmission pool inthe time period T1 (Type 1 discovery or Mode 2 communication), and theRx D2D UEs that are not performing D2D transmission in the cell Areceives and decodes all RB in the D2D reception pool (Type 1discovery). Also, they receive and decode all RB in the D2D receptionpool of the neighboring cells B and C (applied commonly to Type 1discovery and Type 2B discovery).

A method of transmitting and receiving D2DSS for D2D operation betweenUEs located in different cells in an asynchronous network according tothe present disclosure is described in detail hereinafter. A descriptionis made of the operation of the Rx UE according to Type 2B discoverytransmission time and Mode 2 communication transmission timeconfiguration method in the present disclosure hereinafter.

Descriptions are made of the methods of the present disclosure briefly.

(1) Method of Transmitting/Receiving a D2DSS Between D2D UEs:

There are four determinant factors for D2DSS transmission between D2DUEs. First, a D2DSS transmission timing should be determined. Second, aD2DSS Tx UE selection method should be determined. That is, it isrequired to determine a UE which is transmitting the D2DSS. Third, aD2DSS transmission resource selection method should be determined.Finally, a D2DSS transmission time should be determined.

(2) Rx UE Operation According to Type 2B Discovery Tx TimingConfiguration:

The Type 2B discovery message may be transmitted according to thedownlink transmission timing or uplink transmission timing. Accordingly,the operation of the Rx UE may be categorized into one of downlink Txtiming-based Type 2B discovery message transmission operation and uplinkTx timing-based Type 2 discovery message transmission operation.

(3) Rx UE Operation According to Mode 2 Communication Tx TimingConfiguration:

The Rx UE operation may vary depending on the Mode 2 communication Txtiming configuration. The Rx UE may operate distinctly when the Mode 2communication data is transmitted according to the downlink transmissiontiming and when the Mode 2 communication data is transmitted accordingto the uplink transmission timing.

Descriptions are made of the operations of sections (1), (2), and (3)hereinafter in more detail.

Meanwhile, the D2D UE may acquire D2D discovery and D2D communicationresource allocation information from an SIB transmitted by the eNB toperform D2D discovery/communication. That is, the eNB sends the D2D UEslocated in the corresponding cell through the SIB. At this time, theresource allocation information broadcast through the SIB may includethe neighboring cell resource allocation information as well as theserving cell resource allocation information.

A. Resource Allocation Information of Serving Cell:

Discovery type (or communication mode): Information on discovery type(Type 1, Type 2, or both) or communication mode (Mode 1, Mode 2, orboth). In a case of discovery, if this information is received, the D2DUEs can check whether a specific cell supports the Type 1 or Type 2discovery (or both). In a case of communication, the UE can checkwhether the cell support Mode 1 communication or Mode 2 communication orboth of them.

Tx and Rx resource pools information: The Tx and Rx resource poolsinformation may include the information as follows.

a) A case of cell supporting only Type 1 discovery (or Mode 2communication): Since the reception pool is identical with thetransmission pool, one resource pool information is included withoutdistinction.

b) A case of cell supporting only Type 2 discovery (or Mode 1communication): Since the Type 2 discovery message (or Mode 1communication data) is transmitted based on the scheduling of the eNB,no transmission pool exists in Type 2 discovery. Accordingly, thereception pool information is included.

c) A case of cell supporting both Type 1 and Type 2 discoveries: Thetransmission pool information for Type 1 and reception pool informationfor both the Type 1 and Type 2 discovery are included.

d) Transmission pool or reception pool configuration information: Thenumber of subframes constituting the transmission or reception pool, thenumber of RBs constituting a subframe, etc. This information can beexpressed in various formats. For example, The D2D subframeconfiguration information can be informed in the form of a bitmap, e.g.,1011100 . . . . As described above, each bit of the bitmap may set to 1for D2D subframe or 0 for cellular subframe or vice versa. The number ofRBs constituting the D2D subframe can be configured with the start pointand end point of the RB on the frequency axis.

e) Start frame and period of transmission pool or reception pool.

f) After receiving the D2D resource pool information transmitted by theeNB through SIB, the D2D UE has to know whether the corresponding poolis the transmission pool or the reception pool. This information istransmitted through a 1-bit signaling notifying the discovery type orcommunication mode such that the D2D UE performs appropriate operationin the corresponding resource pool. For example, the discoveryType andcommunicationMode as 1-bit size signaling information transmittedthrough SIB may be defined with 1 bit respectively. The discoveryType isset to ‘0’ for Type 1 discovery or ‘1’ for Type 2B discovery. Similarly,the communication mode is set to ‘0’ for Mode 1 communication or ‘1’Mode 2 communication. Such signaling information may be transmittedalong with the resource pool information, e.g., D2D subframeconfiguration information expressed with bitmap and configurationinformation of RBs to be used for D2D discovery or D2D communication inthe D2D subframe.

B. Neighboring Cell Resource Allocation Information:

The neighboring cell resource allocation information may be configuredwith the same information as the serving cell resource allocationinformation with or without some additional information.

1) Discovery type (or communication mode).

2) Transmission pool and reception pool information of neighboring cell.

3) Neighboring cell ID.

For example, the D2D UEs of cell A acquires the serving cell resourceallocation information, neighboring cell ID, and neighboring celltransmission pool and reception pool information corresponding torespective IDs from the SIB transmitted by their serving eNBs. At thistime, the OAM performs coordination such that the resource pools of theserving cell A and neighboring cells are not overlapped.

Although the UEs of the respective cell acquire the resource allocationinformation of the serving and neighboring cells from their servingcells and, in the synchronous network, network synchronization procedureis performed through the network synchronization protocol,synchronization offset of up to +0.5 ms exists. Accordingly, thesubframe level synchronization is not acquired among the cell and thusit is impossible to receive the D2D signal transmitted from neighboringcells.

(1) D2DSS Tx/Rx Method:

First, a description is made of the D2DSS transmission/reception methodamong the aforementioned techniques.

In the synchronization network made up of the cells synchronizedaccurately, it is not necessary to receive the D2DSS. That is, becauseof the synchronization among the neighboring cells, the subframeboundaries match each other. Accordingly, the D2D UEs of the respectivecells that are synchronized with the serving eNB know the subframeboundary of the neighbor cell and thus there is no need to receive theD2DSS. For example, in a case where a cell is split into a plurality ofsectors, the sectors are likely to be synchronized one another. In theLTE system, however, an asynchronous network made up of the cells existand, in such an asynchronous network, it is necessary to transmit theD2DSS for supporting D2D operation between cells.

The higher level entity, e.g., MME, of the eNB may determine whether thenetwork is a synchronous or asynchronous network and may command totransmit the D2DSS or not through S1 interface. If the D2DSS isreceived, the eNBs may command the D2D UEs which they are serving totransmit the D2DSS or not. For example, if D2DSSTransmission=0, thismeans that the cells are synchronized with each other and thus the D2DUEs do not transmit the D2DSS. In contrast, if D2DSSTransmission=1, thismeans that the network is unsynchronized and thus the D2D UEs transmitthe D2DSS.

In order to transmit the D2DSS, it is necessary to determine the D2DSStransmission timing, the D2DSS transmission UE selection method (who),D2DSS transmission resource location method (where), and D2DSStransmission time (when); and operates based on the determinationresult. Descriptions are made the aforementioned 4 factors in serieshereinafter.

A. D2DSS Transmission Timing Configuration Method:

The UE may transmit the D2DSS in compliance with the downlink timing ofthe eNB or the uplink timing of the eNB. In a case of transmitting theD2DSS in compliance with the downlink timing, the D2DSS and the cellularphysical uplink shared channel (PUSCH) or cellular physical uplinkcontrol channel (PUCCH) multiplexed in the same subframe may causeinter-carrier interference (ICI). That is, the D2DSS transmitted incompliance with the downlink timing and the cellular PUSCH transmittedat the uplink reference timing, breaks the orthogonality on the timeaccess due to the different transmission reference timings, therebyresulting in the occurrence of an interference problem at the receiverfront end of the eNB and D2D front end.

Meanwhile, in a case of transmitting the D2DSS in compliance with theuplink timing, it may be difficult for the Rx D2D UEs in the RRC_Idlestate to receive the D2DSS. That is, the D2DSS Tx UE transmits the D2DSSearlier to the same extent as a predetermined offset (N_(TA)·A_(s)) ascompared to the downlink reference timing based on the TA informationreceived from the eNB. The offset time may be referred to as TA, andsince it may change depending on the distance between the D2DSS Tx UEand the eNB and the positions of the D2D Tx UE and D2D Rx UE, the Rx D2DUE in the RRC_Idle state without such information may not configure thereceiving window for receiving the D2DSS. That is, it does not know howmuch it has to advance the receiving window for signal reception.

In order to solve this problem, the serving eNB may transmit to all D2DUEs within the cell the maximum offset of its cell and the maximumoffset of neighboring cells (maximum TA value) using an SIB. If theoffset information is received through the SIB, the D2D UEs adjusts thereceiving window to the same extent as the maximum offset acquiredthrough the SIB to receive the D2DSS.

Whether to apply the downlink timing or uplink timing for transmittingthe D2DSS is communicated to all D2D UEs within the cell when theserving eNB transmits the resource pool information through the SIB(Type 1 discovery) or to a specific UE supposed to transmit the D2DSSthrough dedicated RRC signaling. For example, assuming that the servingeNB supports three discovery (or communication) resource pools, eachresource pool can be the transmission pool or reception pool dependingon whether it is the Type 1 discovery or Type 2B discovery. Also, theD2DSS transmitted in each resource pool may be transmitted in compliancewith the uplink transmission timing or downlink transmission timing. Atthis time, the transmission reference timing of the D2DSS may besignaled (D2DSStiming). That is, the transmission reference timing ofthe D2DSS in the pools 1 and 2 which is set to downlink (D2DSStiming=0)and the transmission reference timing of the D2DSS in the resource pool3 which is set to uplink (D2DSStiming=1) may be transmitted to all UEswithin the cell using the SIB.

In a case of Type 2B discovery, the UEs transmitting the discoverymessages (i.e., Tx D2D UEs) are in the RRC_Connected state. Since theeNB knows the Tx D2D UE in the cell, it commands the corresponding D2DUEs to transmit the D2DSS through dedicated RRC signaling along with thetransmission reference timing of the D2DSS.

B. D2DSS Tx UE Selection Method (Who):

A description is made of a case where all D2D UEs transmit the D2DSS.

First, suppose that all UEs located in cell A transmit the D2DSS. TheD2DSS transmitted by the UE located at the cell center (near the eNB)may not be received by the UEs located in the neighboring cells. Thismeans that the D2DSS transmission causes unnecessary power consumption.This problem becomes worse when the D2DSS isFrequency-Division-Multiplexed (FDM'ed) with the cellular data andcontrol information or D2D discovery message. The reason is that D2DSStransmit power control is performed to prevent the D2DSS transmissionfrom incurring the ICI or in-band emission problem to the cellular dataand control information or D2D discovery message. Since the D2D transmitpower control decreases the D2DSS transmission coverage, it may be worsefor all D2D UEs to transmit the D2DSS.

Second, suppose that specific D2D UE transmits the D2DSS. When the D2DSSTx transmit power control is not considered in Type 1 discovery, the D2DUE may measure the reference signal received power (RSRP) of the servingeNB. If the RSRP of the serving eNB is greater than threshold X dBm,i.e., if the distance from the eNB is greater than a predeterminedthreshold D m, the D2D UE transmits the D2DSS. If the D2DSS transmitpower control is performed in Type 1 discovery, the D2D UE maycalculated the transmit power based on the RSRP (open-loop powercontrol). The transmit power of the D2D UE is greater than Y dBm, it cantransmit the D2DSS. The threshold values may include X dBm, D m, and YdBm. These threshold values are transmitted from the eNB to the D2D UEswithin the cell through an SIB and are determined differently dependingon the radius of the cell.

Meanwhile, the UE performing the Type 1 discovery performs the open-looppower control to minimize the in-band emission or ICI problem occurringto the cellular uplink. Accordingly, it may be specified for the Tx D2DUE to transit the D2DSS when the transmit power of the Tx D2D UE isgreater than a predetermined threshold PTH in the Type 1 discovery. Atthis time, the threshold value PTH is transmitted to the UE through anSIB. Also, the UEs in the RRC_Connected state among the D2D UEsparticipating the Type 1 discovery acquire the TA information, it can bespecified for the corresponding UEs to transmit the D2DSS when the TAvalue which the UEs in the RRC_Connected state have is greater than apredetermined threshold value.

In Type 2 discovery, however, the D2DSS transmission UE is determined bythe eNB. For example, the eNB may check the TA information of the D2DUEs in the RRC_Connected state and, if the TA value of a UE is greaterthan a predetermined threshold value, command the corresponding UE totransmit the D2DSS. In a case where the transmit power is controlled forD2DSS transmission, the eNB may notify the D2DSS transmission UE of thetransmit power value through RRC signaling, or the UE uses the transmitpower used for previous cellular uplink transmission or D2D transmissionor boosts the transmit power up to Z dB based on the previous transmitpower value. At this time, the Z dB power boosting value is communicatedto the D2DSS transmission UE through RRC signaling or using the downlinkcontrol information (DCI) of the physical downlink control channel(PDCCH).

Even in the Type 2 discovery, the D2DSS transmission UE may bedetermined autonomously by the UE, instead of the eNB, as in the Type 1discovery. At this time, the threshold values for determining the UE maybe communicated from the eNB through an SIB. For example, theRRC_Connected UEs which know their TA values determine whether totransmit the D2DSS autonomously using the TA threshold value acquiredthrough the SIB.

C. D2DSS Tx Resource Location Method (where):

A description is made of Type 1 discovery. The objective of transmittingthe D2DSS is to make it possible for a UE located in the neighboringcell to find the start time of the serving cell resource pool (startsubframe). Accordingly, the D2DSS may be transmitted at the firstsubframe in the transmission resource pool of the serving cell.

A description is made of Type 2 discovery. There can be differentscenarios depending on whether a certain cell supports only the Type 2Bdiscovery or both the Type 1 and Type 2B discoveries. Descriptions aremade of a case where only the Type 2 discovery is supported and a casewhere both the Type 1 and Type 2 discoveries, respectively.

First, a description is made of a case of supporting only the Type 2Bdiscovery. Like the Type 1 discovery, since the objective oftransmitting the D2DSS is to make it possible for the UE located in aneighboring cell to fine the start point of the serving cell resourcepool, the D2DSS may be transmitted at the first subframe in thereception pool of the serving cell. In this case, D2DSS transmission andreception problems may occur in view of the D2DSS transmission UE of theserving cell and the D2DSS reception UE of the neighboring cell.

1) Problem at D2DSS Tx UE:

There can be a scenario in which the transmission time of the D2DSS isbased on the downlink reference timing of the serving cell and the Type2B discovery signal is transmitted based on the uplink reference timing(based on TA information of the UE). In this scenario, assuming that then^(th) subframe is the first subframe of the reception pool, the UEattempting to transmit the D2DSS at the n^(th) subframe and thediscovery signal at the (n+1)^(th) subframe consecutively has to give upthe D2DSS transmission at the n^(th) subframe or the Type 2D discoverysignal transmission at the (n+1)^(th) subframe, if the TA value isgreater than 1 symbol. This is because the symbols at the ending part ofthe n^(th) subframe and the symbols at the beginning part of the(n+1)^(th) subframe may collide due to the difference between thetransmission reference timings of the n^(th) and (n+1)^(th) subframes.

In order to solve this problem, the eNB does not allocate the Type 2Bdiscovery signal resource to the UEs, which transmits the D2DSS at then_(th) subframe, at the (n+1)^(th) subframe. In a case where thepriority of the Type 2B discovery signal (e.g., service priority ordelay time priority) is high, the eNB may schedule the Type 2B discoverysignal at the (n+1)^(th) subframe but not the D2DSS at the n^(th)subframe.

2) Problem at D2DSS Rx Terminal:

There can be a scenario in which the transmission time of the D2DSS isbased on the downlink reference timing of the serving cell and the Type2B discovery signal is transmitted based on the uplink reference timing(based on TA information of the UE). In a case of receiving the D2DSS atthe n^(th) subframe and the Type 2B discovery signal at the (n+1)^(th)subframe, significant ICI or inter symbol interference (ISI) may occurat the (n+1)^(th) subframe. In order to solve this problem, is can beconsidered to us a gap wide enough. In the LTE system, it is specifiedto support the cell radius up to 100 km and, since the differencebetween the uplink and downlink reference timings is large in such acell with a large cell radius, it may be necessary to use a gapcorresponding to 8 or more SC-FDM or OOFDM symbols to solve the ICI/ISIproblem.

However, the use of such a long gap is likely to cause resourceutilization inefficiency, when the n^(th) subframe is allocated forD2DSS transmission, it can be considered to allocate the (n+1)^(th)subframe for cellular data and control information transmission, and the(n+2)^(th) subframe for the Type 2B discovery signal transmission. Atthis time, the eNB may schedule the UE with a large TA at the (n+1)^(th)subframe to mitigate the interference of the cellular data transmittedat the (n+1)^(th) subframe to the discovery signal scheduled at the(n+2)^(th) subframe.

Meanwhile, since the Rx UE does not know the TA value of the Type 2Bdiscovery signal transmitted from the neighboring cell, it does not knowthe receiving window. Accordingly, The Rx UE has to receive the signalunder the assumption of the receiving window long enough. In a casewhere the cell radii of the neighboring cells differ from each other, itmay be difficult for the UE to configure the receiving window.

Accordingly, it may be possible for the serving cell of the D2DSStransmission UE to communicate the maximum TA value using the physicalD2D synchronization channel (PD2DSCH) at the subframe carrying theD2DSS. The Rx UE which has acquired the time/frequency synchronizationby receiving the D2DSS receives and decodes the PD2DSCH, adjusts thereceiving window based on the TA information of the neighboring cellindicated by the PD2DSCH, and receives the Type 2B discovery signal. Theserving eNB may notify the D2D UEs location in its cell of the TAinformation of the neighboring cell through the SIB. That is, theserving eNB can communicate the maximum TA value in the cell-specificresource pool information through the SIB. If the SIB is received fromthe serving eNB, the UE receives the D2DSS in the resource pool of theneighboring cell and adjusts the receiving window based on the TA valueindicated in the SIB. This method facilitates the operation of the RxUE. Meanwhile, the eNB notifies all D2D UEs within its cell of themaximum TA value through the PD2DSCH.

A description is made of a case of supporting both the Type 1 and Type2B discoveries.

In a case of supporting both of the two types, the transmission resourcepool for Type 1 discovery may be positioned in the first half or thelast half of the reception resource pool.

First, in a case where the transmission resource pool for Type 1discovery is located in the first half of the reception resource pool:

Since the D2DSS is transmitted in the first frame of the transmissionpool for the Type 1 discovery, the Rx UE can find the start point of thereception pool based on the corresponding D2DSS. Accordingly, it is notnecessary to transmit D2DSS for Type 2B. However, the Type 1 discoverysignal is transmitted in compliance with the downlink reference timing,and the Type 2B discovery signal is transmitted in compliance with theuplink reference timing. For efficient operation of the Rx UE, itcommunicates the TA information of its cell, i.e., maximum TA supportedby its cell, through PD2DSCH.

Second, in a case where the transmission resource pool for Type 1 islocated in the last half of the reception resource pool:

The D2DSS for the Type 2B discovery and the D2DSS for Type 1 discoverycan be used independently. That is, the sequences used in the D2DSS forType 2B and D2DSS for Type 1 differ from each other. By using this, theRx UE can acquire the start point of the reception pool and the startpoint of the transmission pool.

Meanwhile, it is possible to acquire the start point of the receptionpool without transmitting additional D2DSS for Type 2B discovery. Inorder to achieve this, the D2DSS transmission UE transmits the PD2Dschincluding the offset value from the D2DSS subframe for Type 1 discovery.

D. D2DSS Tx Timing Determination Method (when):

The D2DSS transmission timing determination method may be categorizedinto one of a system viewpoint-based determination method and a UEviewpoint-based determination method. In the former case, the D2DSS istransmitted periodically. That is, the first subframe of thetransmission pool is the D2DSS transmission resource in the Type 1discovery, and the first subframe of the reception pool is the D2DSStransmission resource in the Type 2B discovery. Since the discoveryresources are allocated periodically, the D2DSS is transmittedperiodically from the viewpoint of the system.

The D2DSS transmission timing may be determined from the viewpoint ofthe UE. In the latter case, the periodic D2DSS transmission may causeunnecessary power consumption of the UE. For example, if the UE movesfrom the cell edge to the cell center, although it transmits the D2DSSperiodically, the neighboring cell UEs do not receive the D2DSS.Accordingly, each UE transmits the D2DSS only when a condition isfulfilled using the aforementioned D2DSS transmission UE selectionmethod.

The present disclosure is described in more detail hereinafter withreference to the accompanying drawings.

Specifically, FIGS. 1 through 15, discussed below, and the variousembodiments used to describe the principles of the present disclosure inthis patent document are by way of illustration only and should not beconstrued in any way that would limit the scope of the disclosure. Thoseskilled in the art will understand that the principles of the presentdisclosure may be implemented in any suitably arranged communicationssystem. The terms used to describe various embodiments are exemplary. Itshould be understood that these are provided to merely aid theunderstanding of the description, and that their use and definitions inno way limit the scope of the present disclosure. Terms first, second,and the like are used to differentiate between objects having the sameterminology and are in no way intended to represent a chronologicalorder, unless where explicitly stated otherwise. A set is defined as anon-empty set including at least one element.

FIG. 1 A is a diagram illustrating resource allocation for an inter-cellD2D discovery in an LTE D2D system according to an embodiment of thepresent disclosure.

Referring to FIG. 1A, resources 101, 102, 103, 104, 110 and 120 areillustrated, where the resources 101, 102, 103 and 104 are for LTEcellular communication of UEs located in a cell (e.g., a wide areanetwork (WAN)) and the resources 110 and 120 are for D2D communicationof the UEs are TDM'ed.

Each of the resources 110 and 120 allocated for D2D communication iscomposed of M subframes that may be designated for Tx resource pools 111and 121 and Rx resource pools 112 and 122.

In FIG. 1A, the resource 110 which is supposed to be allocated to theD2D UE appearing first, may be an Rx resource pool. A part of the D2Dresource pool may be configured as a Type 1 Tx resource pool 111, andthe remaining part 112 may be allocated as Type 2B resource. In thiscase, since there are UEs supporting only Type 1, the UEs supportingonly Type 2, and the UEs supporting both the types 1 and 2 in thecoverage area of the

If the eNB has allocate the resource as shown in FIG. 1A, then it maysend the UE the information including an offset 141 for indicating astart point of the D2D resource and a discovery period 142 of the D2Dresource through an SIB as described above. For example, the serving eNBmay send all D2D UEs connected to its cell the D2D discovery resourcepool information through the SIB. The UE can check the position of theD2D resource as depicted in FIG. 1A and acquire the information on theoperation mode supported in the discovery period and allocated resource.Descriptions are made of the information included in the SIB which isbroadcast by the eNB for all D2D UEs located in the cell.

(1) discoveryPoolType: Information indicating Tx pool or Rx pool.

(2) discoveryPeriod: Period of D2D resource pool.

(3) discoveryStartPRB, discoveryEndPRB: Frequency axis resourceinformation in D2D resource pool.

(4) discoveryOffset: Information indicating start point of D2D resourcepool (position of start subframe).

In more detail, the discoveryPoolType is the information indicatingwhether the type of the resource is Tx pool or Rx pool for use indetermining whether it is the resource pool for Type 1 discovery or Type2 discovery (hereinafter, Type 2 discovery means Type 2B discovery). Forexample, if the discoveryPoolType set Tx pool is signaled, the D2D Tx/RxUEs determine that the corresponding pool is the resource pool for theType 1 discovery. Otherwise, if discoveryPoolType set to Rx pool issignaled, the corresponding resource pool may be the resource poolsupporting only the Type 2 discovery.

As exemplified in FIG. 1A, if the Type 1 discovery resource pool and theType 2B discovery resource pool are TDM'ed or FDM'ed,discoveryPoolType=both may be signaled.

As exemplified in FIG. 1A, the Rx pool (Type 1 discovery+Type 2discovery) made up of M subframes and the Tx pool (Type 1 discovery)made up of K subframes may be signaled. In another embodiment,discoveryPoolType=Type 1, discoveryPoolType=Type 2, ordiscoveryPoolType=both is signaled directly. Depending on thediscoveryPoolType, discoveryPeriod, discoveryStartPRB, discoveryEndPRB,and discoveryOffset may be set differently.

The subframes constituting the D2D resource pools are transmitted in theform of a bitmap, and FIG. 1A shows an example of using M subframes areall used for D2D. Accordingly, the bitmap representing the subframesconstituting the resource pool becomes ‘11111111 . . . ’.

Meanwhile, the discoveryOffset is the information indicating thesubframe position from which the D2D source pool starts. For example,the serving eNB may communicate the start point of the D2D resource poolbased on its 0th SFN.

FIG. 1B is a diagram illustrating a bitmap representing a resource poolin an intra-cell D2D discovery according to an embodiment of the presentdisclosure.

Referring to FIG. 1B, a D2D resource 110 and LTE resources 101 and 102(e.g., WAN) are illustrated, where the D2D resource 110 may bepositioned between the LTE resources 101 and 102 as described above. TheD2D resource 110 may be divided into D2D subframes and WAN subframes forallocation to the LTE UEs.

For example, if the D2D subframe is represented by 1 while the WANsubframe is represented by 0, the bitmap may be configured in variousformats. FIG. 1B shows the bitmaps configured as ‘11011011 . . . ’ and‘10111110 . . . ’. In FIG. 1B, the shaded area denotes the D2D subframesand the non-shaded area denotes the WAN subframes.

In a case of supporting only the Type 1 discovery, the Tx pool consistsof M subframes. In a case of supporting only the Type 2B discovery orthe Type 1 discovery and Type 2B discovery are TDM'ed or FDM'ed, the Rxpool consists of M subframes.

FIGS. 2A and 2B are diagrams illustrating a resource allocation forintra-cell D2D communication in an LTE D2D system according to variousembodiments of the present disclosure.

Referring to FIG. 2A, WAN resources 201, 202, 203, 204, 205 and 206,scheduling assignment (SA) resource pools 211 and 212 and data resourcepools 221 and 222 for D2D communication are illustrated, where the WANresources 201, 202, 203, 204, 205 and 206 are arranged from a startpoint of SFN 0 on a time axis, and the SA resource pools 211 and 212 anddata resource pools 221 and 222 for D2D communication are arrangedbetween them. The resource arrangement information may be configured bythe eNB and provided to all UEs within the coverage area of thecorresponding eNB through an SIB.

As shown in FIG. 2A, the data offset may be the value of indicating thestart time of the data period in association with the start point of SNF0. As shown in FIG. 2B, the data offset may indicate a number ofsubframes from the start time of the SA subframe in another example.FIGS. 2A and 2B differ from each other in start point of the dataoffset. That is, the data offset indicates the distance from the startpoint of the SFN 0 in FIG. 2A but the distance from the start point ofthe SA resource pool in FIG. 2B. As shown in FIGS. 2A and 2B, an SAperiod 241 for SA transmission and a data period 251 for datatransmission are differentiated from each other. At this time, each ofthe SA pools 211 and 212 consists of M subframes, and each of the datapools 221 and 222 consists of N subframes.

As described above, the serving eNB transmits the SA and data resourcepools information for D2D communication to all UEs connected to its cellthrough the SIB. At this time, the resource pool information carried inthe SIB may include the information as follows.

(1) communicationPoolType: Information whether the SA is the Mode 1 SAor Mode 2 SA.

(2) saPeriod: Period of SA resource pool.

(3) SA bitmap: Information indicating structure of SA subframe.

(4) saStartPRB, saEndPRB: Information of resource on frequency axis inSA resource pool.

(5) saOffset: Information indicating start point (start subframe) of SAresource pool.

(6) dataPeriod: Period of data resource pool.

(7) data bitmap: Information indicating structure of data subframe.

(8) dataStartPRB, dataEndPRB: Information of resource on frequency axisin data resource pool.

(9) dataOffset: Information indicating start point (start subframe) ofdata resource pool.

The subframes constituting the SA and data resource pool are transmittedin the form of a bitmap as in the discovery. FIGS. 2A and 2B show casesof using M subframes all for SA as denoted by reference number 211 and212 and N subframes all for data as denoted by reference numbers 221 and222. Accordingly, the bitmap representing the subframes constituting theSA and data resource pool is shown as 11111111 . . . . If the bitmap ofSA and data includes 0 (e.g., 10110 . . . , etc.), 0 indicates thesubframe without SA or data for D2D communication.

The dataOffset is the information indicating the position of thesubframe at which the SA resource pool starts, and the dataOffset is theinformation indicating the position of the subframe at which the dataresource pool starts. For example, the serving eNB may communicate thestart point of the SA and data resource pools in association with theSNF 0 as shown in FIG. 2A or a predetermined SFN. Also, the dataOffsetmay indicate the start point of the data subframe in association withthe SFN 0 as shown in FIG. 2A or in association of the start point ofthe SA resource pool as shown in FIG. 2B.

In a case where the SA resource pool is subsequently followed by thedata resource pool (if there is no WAN between the SA resource pool andthe data resource pool in FIGS. 2A and 2B), it may not necessary to usedataOffset indication through SIB.

The resource allocation for inter-cell D2D communication is similar tothat for intra-cell D2D communication. The serving eNB transmitsneighboring Cell IDs, communication PoolType, SA resource poolinformation (saPeriod, saBitmap, saStartPRB, saEndPRB, and saOffset),and data resource pool information (dataPeriod, dataBitmap,dataStartPRB, dataEndPRB, and dataOffset). Further, the D2D UEsconnected to the serving eNB (RRC_Connected state) and the D2D UEscamped on the serving eNB (RRC_Idle state) receive the informationthrough the SIB. The UE transmits signals in the resource pool of theserving eNB and receives SA and data in the resource pool of theneighboring eNBs based on the received information.

FIGS. 3A and 3B are diagrams illustrating an SA and data resourceallocations for D2D communication according to various embodiments ofthe present disclosure.

Referring to FIG. 3A, a diagram illustrating the resource allocation forMode 1 communication is provided, and referring to FIG. 3B a diagramillustrating the resource allocation for Mode 2 communication isprovided.

Referring to FIG. 3A, PUCCHs are mapped to subframes positioned at bothedges of a frequency band, i.e., all of the subframes indicate thefrequency indices 1 and S, and at least one guard band may be positionbetween the frequencies to which the PUCCH and SA are mapped. Also, theSA information may be transmitted on the frequency resource with theexception of the part corresponding to PUCCH and guard band for apredetermined number of subframes of the SA resource pool 310.

Meanwhile, the Mode 1 communication is allocated the SA resource pool310 for M subframes and, subsequently, the data resource pool 320 for Nsubframes. In FIG. 3A, the subframes at both boundary frequencies of thedata resources, i.e., the subframes on the frequencies with indices 1and S, are occupied by PUCCH, and the data resource may include the WANresource and D2D resource. In FIG. 3A, the shaded area in the dataresource pool 320 indicates the D2D data resource.

Referring to FIG. 3B, a WAN resource (support) 330 is allocated betweenthe SA resource pool 310 and the D2D data resource pool 320, and the D2Ddata resource pool 320 is used dedicatedly so as not to be used for WANcommunication. Accordingly, the boundary frequencies of the D2D resourcepool 320 consisted of N subframes are allocated for PUCCH, and guardbands are interposed between the D2D data resource and PUCCH resources.

In the D2D communication based on the resource configuration as shown inFIGS. 3A and 3B, the SA which is transmitted by the D2D Tx UE mayinclude the control information as follows:

(1) Frequency hopping indicator;

(2) Frequency resource;

(3) MCS (Modulation and Coding Scheme);

(4) T-RPT (Time-Resource Pattern of Transmission);

(5) TA (Timing Advance); and

(6) ID (Identifier).

In the SA information, the frequency hopping indicator is 1-bitindication information indicating whether there is frequency hopping inthe data region 320 following the SA transmission. The frequencyresource indicates the frequency position and sized of the data resourceon the frequency axis. The T-RPT indicates the D2D data transmissionpattern on the time axis in the form of a bitmap. For example, if thebitmap of the T-RPT included in the SA is 1101110, 1 indicates thesubframe carrying D2D data and 0 indicates the subframe carrying no D2Ddata. The TA is the information transmitted for adjusting the receivingwindow in the SA when the D2D data transmitted in compliance with theuplink reference timing is received. That is, the TA included in the SAis transmitted form the eNB to the Tx D2D UE for use in adjusting thereceiving window when the Rx D2D UE receives data.

The ID included in the SA is a “destination ID” as the ID of a specificgroup or a UE belonging to the group. At this time, the group is thegroup set for supporting public safety such as a firefighter group,police group, and SWAT group. For example, if the SA Tx UE transmits thedata targeted to the firefighter group, the destination ID included inthe SA may be the fighter group ID.

In the Mode 1 communication, the eNB communicates the SA resource poolstart point (saOffsetIndicator) and period (saPeriod) on the time axisand start point (saStartPRB) and end point (saEndPRB) on the frequencyaxis and the entire PRB size (saNumPRB) occupied by the SA on thefrequency axis in a subframe. The reason for transmitting suchinformation through SIB is to make it possible for the RRC_Idle UEs toreceive the SA.

The UE intending transmission through Mode 1 communication, among theUEs decoded the SA resource pool information broadcast through the SIB,should be in the RRC_Connected state. The UE in the RRC_Idle state hasto transition to the RRC_Connected state through the random accessprocedure. The Tx D2D UE transitioned to the RRC_Connected state isallocated the time/frequency resource through SA from the eNB. At thistime, the time/frequency resource allocation information of the SA istransmitted through the downlink control information (DCI) included inthe PDCCH. At this time, in order to check whether the resource isallocated for D2D transmission or cellular uplink transmission, the DCIfor D2D resource allocation is scrambled with D2D-dedicated radionetwork temporary identifier (RNTI).

The Mode 1 communication is characterized in that no resource for datatransmission exists. That is, the eNB notifies the Tx D2D UE of thetime/frequency position information of the data transmission resource(not data resource pool information) using DCI other than SIB. Afterdecoding the DCI received from the eNB, the Tx D2D UE checks thetime/frequency position for data transmission and transmits the SAincluding the corresponding information. The Rx D2D UE which hasacquired the SA resource pool information through the SIB decodes the SAin the corresponding SA resource pool to check the data resourcetime/frequency position in the SA and then decodes the data.

In the Mode 2 communication, like the Mode 1 communication, the eNBbroadcasts the SA resource pool information (saPeriod, saNumPRB,saStartPRB, saEndPRB, saOffsetIndicator, saSubframeBitmap, etc.) throughan SIB. Unlike the Mode 1 communication, the eNB broadcast the datatransmission resource pool information through the SIB too. That is, theSIB may carry the data resource pool period, data resource poolStartPRB, data resource pool EndPRB, data resource pool NumPRB, dataresource pool OffsetIndicator, data resource pool SubframeBitmap, etc.

In the Mode 2 communication, the eNB only communicates the SA and dataresource pool information and thus the UE selects the resource to use inthe SA and data resource pool in a distributed manner. That is, the TxD2D UE selects the SA resource randomly or in an energy sensing-basedmanner from the SA resource pool and determines the time-resourcepattern of transmission (T-RPT) and frequency resource position for datatransmission. The T-RPT and frequency resource position for datatransmission is applied in the data resource pool. The Rx D2D UE decodesthe SA and data resource pool information received from the eNB throughthe SIB. The UE receives the SA in the SA and data resource pool anddecodes the corresponding data in the data resource pool using the T-RPTand frequency resource position included in the SA.

FIG. 3A shows a resource allocation in the Mode 1 communication in whichthe eNB communicates the time/frequency resource for SA transmission ofthe Tx D2D UE in the SA resource pool transmitted through the SIB usingthe DCI of PDCCH. That is, the eNB instructs the Tx D2D UE 1 to transmitthe SA 1 on the (N−1)^(th) frequency resource in the subframe 1 and theTx D2D UE 2 to transmit the SA 2 on the (N−3)^(th) frequency resource inthe subframe 2. In order to solve the half-duplex problem in the D2Dcommunication, it is required for the UEs belonging to the same group todo not transmit the SA in the same subframe.

The half-duplex problem can solved as described above (the eNB schedulessuch that the UE 1 transmits SA at subframe 1 and UE 2 transmits SA atsubframe 2). For robust transmission of SA, it is possible to transmitthe SA repeatedly. The repetitive transmission occurs K times in the SAresource pool where K may be set to a value equal to or less than thenumber of subframes of the SA resource pool. Although only the SA, GuardBand, and PUCCH are depicted in FIG. 3A, The SAs may be FDM'ed with thecellular data WAN in the SA pool. At this time, the Guard band is usedto avoid the ICI problem occurring between the PUCCH transmitted incompliance of the uplink reference timing based on TA and the SAtransmitted in compliance with the downlink reference timing. Meanwhile,the time/frequency resource for D2D data transmission is allocated bythe eNB through DCI of PDCCH, and the Tx D2D UE includes the allocatedtime/frequency resource information in the T-RPT and data RB allocationincluded in the SA and decodes the data at the correspondingtime/frequency resource position.

FIG. 3B shows a resource allocation in the Mode 2 communication in whichthe Tx D2D UE selects resource for SA transmission in the SA resourcepool indicate in the SIB. At this time, the resource selection can beperformed randomly or in an energy-based manner. In a case of randomselection, all SA transmission resources for repetitive transmission maybe selected randomly in the SA resource pool. That is, in order to solvethe half-duplex problem in the SA resource pool consisted of M subframesand transmit SA robustly, the same SA can be transmitted K timesrepeatedly. At this time, K may be set to a value equal to or less thanM. In this case, the SA resource for K transmissions may be selectedrandomly at every transmission timing. Also, the frequency position ofthe SA resource selected in the subframe 1 may be mapped to thefrequency resource positions of the (K−1) repetitive transmissionsimplicitly. Such mapping information may be specified in the standard orconfigured by the eNB through SIB. After selecting the SA resource, theTx D2D UE selects the T-RPT indicating the frequency resource and timeresource for data transmission and transmit the corresponding SA in theselected T-RPT.

At this time, the frequency resource and T-RPT selection for datatransmission is made regardless of the SA resource selection or mappedto the SA resource. In a case of determining regardless of the SAresource selection, the Tx D2D UE which has selected the SA resource maydetermine the frequency resource and T-RPT for data transmissionrandomly. Otherwise, if the data resource is mapped to the SA resource,the position of the SA resource may be mapped to the position of thetime/frequency position of the data resource implicitly.

In the Mode 2 communication, the time/frequency positions of the SA anddata resource pools are transmitted to all D2D UEs within the cellthrough the SIB. The SA and data resource pools may not be consecutiveas shown in FIG. 3B. That is, there may be the resource 330 for uplinkcellular transmission between the SA resource pool 310 and data resourcepool 320.

In FIG. 3B, it is assumed that the Mode 2 data is transmitted incompliance with the downlink reference timing. Unlike FIG. 3A, a GuardBand may exist between the PUCCH and Mode 2 data in the data resourcepool. If the Mode 2 data is transmitted in compliance with the uplinkreference timing like the Mode 1 data of FIG. 3A, the Guard Band is notnecessary in the data resource pool of FIG. 3B.

FIGS. 4A and 4B are diagrams illustrating an SA and a data resourceallocation in a case where Mode 1 and Mode 2 SAs are FDM'ed according tovarious embodiments of the present disclosure.

FIGS. 4A and 4B show other examples of the SA and data resourceallocation according to various embodiments of the present disclosure inwhich the SA for Mode 1 communication and the SA for Mode 2communication are FDM'ed in the same SA resource pool 410.

Referring to FIG. 4A, a Mode 1 SA resource pool 411 and a Mode 2 SAresource pool 412 are illustrated, where the Mode 1 SA resource pool 411and the Mode 2 SA resource pool 412 are identical on the time axis butdifferent in the frequency axis. The frequency resource of the Mode 1 SApool is saStartPRB=S−2, NumPRB1=(S−2)−(R+3)+1, and the frequencyresource of the Mode 2 SA pool 412 has saEndPRB=3, NumPRB2=(R−1)−3+1. Inorder to reduce signaling overhead, the NumPRB1 and NumPRB2 may have thesame value. The frequency resource position of the SA pool may beallocated by the eNB through a SIB. The time axis resource (subframes)410 of the common SA pool of Mode 1 and Mode 2 is informed usingsaPeriod, saOffsetIndicator, and saBitmap broadcast in the SIB for theD2D UEs within the cell.

The Mode 1 communication data starts right after the end of the Mode 1SA pool 411, but the Mode 2 communication data may not start right afterthe end of the Mode 2 SA pool 412. The Mode 2 data has the frequencyaxis pool defined with dataStartPRB, dataEndPRB, and dataNumPRB and thetime axis resource pool defined with dataPeriod, dataOffsetIndicator,and dataBitmap (Mode 2 data pool) 430. However, the Mode 1 data has notdata resource pool defined. The Mode 2 communication may operate in afallback mode in the coverage area of the eNB. That is, although it istypical that the D2D communication operates in Mode 1 in the eNBcoverage, the D2D communication may operate in Mode 2 in a specialsituation, e.g., the synchronization signal of the eNB may not bereceived according to an eNB command for a predetermined time period inan emergency operation. For such case, there may be a need of thededicated resource for Mode 2 communication. Accordingly the resourceallocation method of FIG. 4A is appropriate for supporting such fallbackmode operation.

The D2D UEs decodes the communicationPoolType received through the SIBfrom the eNB to acquire the resource pool information on the respectivemodes, i.e., the UE receives and decodes the SIB to acquire theinformation on the time/frequency position of the Mode 1 SA pool andtime/frequency position of the Mode 2 SA pool. The communicationPoolTypetransmitted in the SIB may be managed implicitly or explicitly with1-bit or 2-bit signaling as follows.

(1) Implicit management of communicationPoolType: The frequency resourceindicated by asStartPRB and saNumPrb is recognized as the Mode 1 SAresource pool 411, and the frequency resource indicated by saEndPRB andsaNumPRB is recognized as the Mode 2 SA resource pool 412. The reversecase is possible and, in this case, the saNumPRB for Mode 1 and saNumPRBfor mode 2 may be set to different values or the same value for reducingsignaling overhead.

(2) Explicit management of communicationPoolType: It is carried in theSIB transmitted by the eNB as 1-bit signal set to 0 for Mode 1 SAresource or 1 for Mode 2 SA resource pool. Also, this parameter may beset to 1 for Mode 1 SA resource and 0 for Mode 2 SA resource pool. Ineach communication mode, saStartPRB, saNumPRB, and saEndPRB may exist.Although not shown in FIGS. 4A and 4B, if such a signaling is used,there may be two SA resource pools for each of Mode 1 and Mode 2. Thatis, there may be 4 frequency axis resource pools in the same time axisSA pool.

For example, there is two resource pools indicated by saStartPRB andsaNumPRB in the Mode 1 resource pool and two resource pools indicatedidentically in the Mode 2 resource pool. The frequency axis resourcepools in the same mode can be used in different groups. For example, thefirefighter group and the police group may use different frequency axisresource pools in Mode 2.

In a case of suing 2-bit signaling, it can be indicated as thecommunicationPooolType={00, 01, 10, 11}. In FIG. 4A, if the Mode 1 SAresource pool is indicated by 0 and the Mode 2 SA resource pool isindicated by 1, the SA resource pool is indicated by signalingcommunicationPoolType=01.

In the FIG. 4A, the Guard Band is set to avoid the ICI problem occurringbetween RBs using different transmission reference timings. For example,the SAs for Mode 1 and Mode 2 are transmitted in compliance with thedownlink reference timing in the SA resource pool while the cellularsource (WAN) is transmitted in compliance with the downlink referencetiming. Likewise, in a case where the Mode 2 data is transmitted incompliance with the downlink reference timing, the guard band is set toavoid the ICI problem with the cellular resource (WAN) or Mode 1 datatransmitted in compliance with the uplink reference timing.

In a case where the Mode 2 data is transmitted in compliance with theuplink reference timing, it is not necessary to set the Guard band.Meanwhile, in the SA resource pool, the cellular resource (WAN) may beused as FDM'ed with the Mode 1 SA resource pool and Mode 2 SA resourcepool or not. In this case, it is not necessary to set the guard bandexplicitly between the Mode 1 SA resource pool and the cellular resource(WAN) and between the Mode 2 SA resource pool and the cellular resource(WAN).

Referring to FIG. 4B, another embodiment of the present disclosure isillustrated, in which Mode 1 SA and the Mode 2 SA are FDM'ed unlike acase of FIG. 4A in which the Mode 1 SA pool and Mode 2 SA pool arearranged separately on the frequency axis. Thus, there is no need totransmit the communicationPoolType in the SIB. In a case of using theMode 1 communication and Mode 2 communication, the Tx D2D UEs operateper Mode in the SA resource pool. That is, all D2D UEs receives the timeaxis information of the SA resource pool which includes saPeriod,saOffsetIndicator, and saBitmap; and the frequency axis information ofthe SA resource pool which includes saStartPRB, saEndPRB, and saNumPRB.

Among the D2D UEs received the information, the Tx D2D UE using the Mode1 communication is allocated the time/frequency resource for SAtransmission through DCI of PDCCH. The SA carries the time/frequencyposition of the SA resource and T-RPT of the data resource included inthe DCI information frequency resource allocation, and frequency axissize of the data resource. The Rx D2D UE using the Mode 1 communicationacquires the data resource allocation information included in the SA todecode the data at the corresponding resource position.

The Tx D2D UE using the Mode 2 communication selects resource from theSA and data resource pool indicated in the SIB from the eNBautonomously. For example, the eNB may select the unoccupied SA resourcein the SA resource pool in an energy sensing manner. After decoding allRBs existing in the SA resource pool, the UE may select available SAresource. The UE selected the SA resource available in Mode 2 selectsthe T-RPT and frequency resource for data transmission randomly andtransmits the data along with the SA information. At this time, theT-RPT and frequency resource for data transmission should be defined inthe Mode 2 data resource pool. The Mode 2 communication Rx UE receivesand decodes SA resource and then decodes the data using the T-RPTincluded in the SA resource and frequency resource position of the data.Accordingly, there is no need of the communicationPoolType informationtransmitted in SIB in a case of FIG. 4B.

FIG. 5 is a diagram illustrating a resource allocation for a TDM of anSA and data according to an embodiment of the present disclosure.

Referring to FIG. 5, an SA resource pool 511 and an SA resource pool 512are illustrated, where the SA resource pool 511 is of Mode 1 and the SAresource pool 512 is of Mode 2 and the SA resource pool 511 and the SAresource pool 512 are TDM'ed in a Mode 1 SA pool period 510.

In the Mode 2 communication, the eNB broadcast dataPeriod,dataOffsetIndicator, and dataBitmap in an SIB to communicate the timeaxis information of the data resource pool. At this time, the dataBitmapinformation indicates subframes with mode 2 D2D resource. For example,the dataBitmap set to 1011011 . . . informs that subframe 1, subframe 3,subframe 4, subframe 6, and subframe 7 have mode 2 D2D resource. At thistime, if the Mode 2 Tx UE has selected T-RPT of 10110 randomly for datatransmission, this means that the Tx UE A transmits data at subframe 1,subframe 4, and subframe 6. In order to reduce signaling overhead, thedataBitmap and T0RPt may use repetitive transmission. For example, ifthe Mode 2 data resource pool 513 consists of N subframes, it may bepossible to configure the n-bit default dataBitmap and repeat the n-bitdataBitmap Q times to meet (n×Q)≤N.

Such information is carried in the SIB broadcast by the eNB.Particularly in a case where different SA resource pools 511 and 512 areTDM'ed with a Mode 2 data resource pool 513 as shown in FIG. 5(including 2 or more Mode 2 SA resource pools although not shown in FIG.5), the Mode 2 data resource pool 513 may collide with a subsequent Mode1 SA resource pool 521 or Mode 2 SA resource pool (not shown). In orderto avoid this, it is required to design such that the product of therepetition times of dataBitmap (R) and the size of the dataBitmap (n)becomes less than saPeriod. For example, assuming saPeriod={80, 160,320} ms and dataBitmap size of Mode 2 data=40 ms (40 bits), it isdesigned to be less than R={1, 3, 7} for respective saPeriod values.

FIG. 6 is a diagram illustrating resource allocation for an inter-cellD2D discovery according to an embodiment of the present disclosure.

Referring to FIG. 6, eNBs 601, 602 and 603 are illustrated, where theeNBs 601, 602, and 603 notify D2D UEs connected to their cells of theinformation on the discovery resource pool used in the neighboring cellas well as the information on its own D2D discovery resource poolthrough an SIB.

If such information is received, the UE receives the D2D signaltransmitted in the discovery resource pool of the serving cell and thediscovery resource pool of the neighboring cell. For example, the D2DUEs of cell 1 601 receive the D2D discovery signals transmitted in theD2D discovery resource pools of cell 2 602 and cell 3 603 as well ascell 1 601.

Meanwhile, the D2D discovery Tx UE of cell 1 601 transmits the D2Ddiscovery signal only in the discovery transmission resource poolavailable for its cell but not in the discovery transmission resourcepool of the neighboring cells 602 and 603.

In a case of Type 1 discovery, it is determined that the D2DSS anddiscovery message are transmitted in compliance with the downlinktiming. In a case of Type 2B discovery, however, nothing is determinedyet. In a case of Type 2B discovery, various scenarios can be consideredas shown in table 1.

TABLE 1 D2DSS Tx reference Type 2B discovery message Scenario timing Txreference timing 1 Downlink timing Downlink timing 2 Downlink timingUplink timing 3 Uplink timing Uplink timing

In a case of scenario 1 of table 1, the Type 2B discovery D2DSStransmission reference timing and Type 2B discovery message transmissionreference timing are identical with the Type 1 discovery D2DSS and Type1 discovery message transmission reference timing. Thus, no signaling isrequired for communicating the transmission reference timing. That is,it is not required to transmit the maximum offset information throughSIB or dedicated RRC signaling. This means that it is not necessary forthe Rx D2D UE in the RRC_Idle state to move up the receiving windowearlier than the downlink reference timing which it has received forreceiving the D2DSS or Type 2B discovery message. In a case where theType 2B discovery message is frequency-multiplexed with the cellularPUSCH, significant ICI problem may occur at the reception front end ofthe eNB and D2D reception front end.

In a case of scenario 2, the D2D UE has to know that the Type 2Bdiscovery message is transmitted in compliance with the uplink timing.Such information can be transmitted with a 1-bit signal through SIB ordedicated RRC signaling. That is, all D2D UEs received the SIB withinthe cell know that the discovery message should be transmitted incompliance with the uplink reference timing in the Type 2B discoveryresource pool. Since the Tx D2D UE is in the RRC_Connected state in theType 2B discovery, the eNB may command the Tx D2D UE to transmit theType 2B discovery message in compliance with the uplink referencetiming. At this time, the eNB may command the Tx D2D UE to transmit theD2DSS which is preconfigured to be transmitted according to the downlinkreference timing.

If it is specified to transmit the D2DSS in compliance with the downlinkreference timing always and the Type 2B discovery message in compliancewith the uplink reference timing always in the standard, the informationmay be mapped to the Type 2B resource pool allocation information. Forexample, assuming that the resource pool 1 is used for Type 1 discoveryand the resource pool 2 is used for Type 2B discovery, the Tx D2D UEtransmits the D2DSS and the Type 1 discovery message in compliance withthe downlink resource timing in the resource pool 1. The Tx D2D UE mayalso transmit the D2DSS in compliance with the downlink reference timingand the Type 2B discovery message in compliance with the downlinkreference timing in the resource pool 2. Meanwhile, in order for the RxD2D UEs in the RRC_IDLE state to receive, in the serving cell andneighboring cell, the Type 2 discovery message transmitted in compliancewith the uplink timing, the serving eNB has to communicate the maximumTA information of the serving cell and neighboring cells through SIB orPD2DSCH.

If it is specified to transmit the D2DSS in compliance with the downlinkreference timing always and the Type 2B discovery message in compliancewith the uplink reference timing always in the standard, the informationmay be mapped to the Type 2B resource pool allocation information. Forexample, assuming that the resource pool 1 is used for Type 1 discoveryand the resource pool 2 is used for Type 2B discovery, the Tx D2D UEtransmits the D2DSS and the Type 1 discovery message in compliance withthe downlink resource timing in the resource pool 1. The Tx D2D UE mayalso transmit the D2DSS in compliance with the downlink reference timingand the Type 2B discovery message in compliance with the downlinkreference timing in the resource pool 2.

Meanwhile, in order for the Rx D2D UEs in the RRC_IDLE state to receive,in the serving cell and neighboring cell, the Type 2 discovery messagetransmitted in compliance with the uplink timing, the serving eNB has tocommunicate the maximum TA information of the serving cell andneighboring cells through SIB or PD2DSCH.

Finally, in a case of scenario 3, the D2D UE has to know that the D2DSSand Type 2B discovery message are transmitted in compliance with theuplink timing. It can be transmitted with a 1-bit signal through SIB ordedicated RRC signaling. That is, all D2D UEs received the SIB withinthe cell know that the D2DSS and discovery message should be transmittedin compliance with the uplink reference timing in the Type 2B discoveryresource pool.

Since the Tx D2D UE is in the RRC_Connected state in the Type 2Bdiscovery, the eNB may command the Tx D2D UE to transmit the Type 2Bdiscovery message in compliance with the uplink reference timing. Atthis time, the eNB may command the Tx D2D UE to transmit the D2DSS whichis supposed to be transmitted in compliance with the uplink referencetiming. If it is specified to transmit the D2DSS and discovery messagein the Type 2 discovery in compliance with the uplink reference timingalways in the standard, the information may be mapped to the Type 2Bresource pool allocation information.

For example, assuming that the resource pool 1 is used for Type 1discovery and the resource pool 2 is used for Type 2B discovery, the TxD2D UE transmits the D2DSS and the Type 1 discovery message incompliance with the downlink resource timing in the resource pool 1. TheTx D2D UE may also transmit the D2DSS and discovery message incompliance with the uplink reference timing in the resource pool 2.Meanwhile, in order for the Rx D2D UEs in the RRC_IDLE state to receive,in the serving cell and neighboring cell, the D2DSS and Type 2 discoverymessage transmitted in compliance with the uplink timing, the servingeNB has to communicate the maximum TA information of the serving celland neighboring cells through SIB. The Rx D2D UEs received theinformation search for the start point of the resource pool based on theD2DSS and then receives the Type 2B discovery message. In a case oftransmitting the maximum TA information through the SIB, since the Rx UEdoes not know the TA information of the Tx UE, it should be in the Rxmode for time duration long enough to configure the D2DSS receptionwindow. That is, the Rx UE has to enter the RX mode earlier as much as 9SC-FDM symbols than its downlink reference timing.

Furthermore, referring to FIG. 6 D2D resources 611 and 612 of the cell 1601, D2D resources 621 and 622 of the cell 2 602 and the D2D resources631 and 632 of the cell 3 603 are illustrated, where the D2D resources611 and 612 of the cell 1 601, the D2D resources 621 and 622 of the cell2 602, and the D2D resources 631 and 632 of the cell 3 603 may beseparated with gaps of a few ms. Since the reason for use of the gapshas been described above in association with the phenomenon occurring inthe asynchronous network, detailed description thereof is omittedherein.

FIGS. 7A to 7C are diagrams illustrating D2DSS Tx resource allocationfor an inter-D2D communication according to various embodiments of thepresent disclosure.

Referring to FIG. 7A to 7C, similar to inter-cell D2D discovery, an eNBof each cell notifies D2D UEs connected thereto of SA resource poolinformation and data resource pool information on the communication usedin the neighboring cell as well as the SA resource pool and dataresource pool information on the D2D communication used in its cellthrough an SIB. The UE which has received this information receives theD2D signal transmitted in the SA/data resource pool of the serving celland the SA/data resource pool of the neighboring cell. For example, theD2D UE located in cell 1 601 of FIG. 6 receives the D2D communicationsignal transmitted in the SA/data resource pool of the cell 2 602 andcell 3 603 as well as cell 1 601.

Meanwhile, the D2D communication Tx UE of cell 1 601 transmit the D2Dcommunication signal only in the SA/data transmission resource poolavailable for its cell but not in the communication transmissionresource pool of the neighboring cell. That is, the UE performs onlysignal reception in the transmission resource pool of the neighboringcell.

Unlike the D2D discovery, the D2D communication should be supported in apartial coverage situation. The partial coverage situation is thesituation in which some D2D UEs are located in the coverage area of theeNB while some D2D UEs are located out of coverage area of the eNB. Thismeans that the eNB may be damaged by natural disaster such as earthquakeand tsunami, fire, or terror. In such a situation without presence ofeNB, periodic D2DSS transmission is required for synchronization amongthe UEs. The D2DSS transmission resource may be predetermined as shownin FIGS. 7A and 7B or may be communicated under the configuration of theeNB as shown in FIG. 7C.

First, descriptions are made of the resource structures of FIGS. 7A to7C schematically.

FIGS. 7A to 7C show resource structures identical in transmissionformat. That is, LTE cellular network transmission resources 701, 702,703, 704, 705 and 706, SA resource pools 711 and 712, and data pools 721and 722 are TDM'ed. The resource allocation information to indicate suchresource configuration may be transmitted through an SIB as describedabove. At this time, saOffset 731 indicates a start point of the SAresource pool 711 from SFN 0, and dataOffset 732 indicates the startpoint of the data resource pool 721 from SFN 0. The SA period 741 anddata period 742 indicate the respective discovery periods.

A description is made of the D2DSS transmission hereinafter withreference to FIGS. 7A to 7C. As shown in FIG. 7A, the D2DSS may alwaysbe transmitted at the start point of the SA resource pool. This is acase where the D2DSS is transmitted at first subframes 711 a and 712 aof the SA resource pools 711 and 712. In this case, there is no need ofextra signaling for D2DSS transmission resource configuration. In a casewhere the period of the SA resource pool 741 is long, however, thetransmission period 151 of the D2DSS is prolonged to causesynchronization error, resulting in performance degradation.

Meanwhile, the D2DSS may be transmitted periodically at the start pointof the SA resource pools 711 and 712 and in the D2D data resource pool721 and 722 following the SA as shown in FIG. 7B, as denoted byreference numbers 721 a 1, 721 a 2, 722 a 1 and 722 a 2. For thispurpose, the eNB may signal the transmission period of the D2DSS throughan SIB or dedicated RRC signaling. The UE located in the coverage areaof the eNB may relay the transmission period of the D2DSS throughPD2DSCH to the UEs located out of the coverage area of the eNB. Sincethe method of FIG. 7B can set a short D2DSS transmission period incomparison to the method of FIG. 7A, it is advantageous in terms ofimproving the synchronization performance but disadvantageous in thatthe D2DSS Tx UE cannot receive SA or data even in the subframe carryingthe D2DSS due to the half-duplexing problem. That is, since the D2DSS TxUE cannot receive signals in the Tx mode, it cannot receive the SA anddata transmitted by other D2D UEs in the D2DSS subframe.

In order to overcome this shortcoming, the eNB may configure theresource for D2DSS transmission in the WAN cellular subframe as shown inFIG. 7C. The RBs with the exception of the D2DSS transmission resource(e.g., center 6 RBs) can be used for transmitting cellular uplink dataand control information. That is, since the WAN subframes 701, 702, 703,704, 705 and 706 are used for transmitting cellular data and controlinformation, the D2D UEs do not transmit or receive therein.Accordingly, it is required for the eNB to notify the D2D UEs within thecell of the D2DSS transmission in the WAN subframes 701, 702, 703, 704,705 and 706 and its transmission period through an SIB. For example, theD2DSS may be transmitted in a subframe 701 a predetermined in the firstWAN resource pool 701 as shown in FIG. 7C. Furthermore, the D2DSStransmission resource information should be relayed to theout-of-coverage UEs through PD2DSCH for D2DSS transmission in thecorresponding subframe.

The operations of FIGS. 7A, 7B, and 7C may be performed independently orin various combination. That is, the D2DSS may be transmitted in the SAresource pool and WAN resource pool, or in the SA resource pool, dataresource pool, and WAN resource pool. In order to achieve this, the eNBmay notify the D2DSS Tx UEs of the information on the resource pool forD2DSS transmission through the SIB or dedicated RRC signaling.

In a case where the D2DSS is transmitted in the SA resource pool asshown in FIG. 7A, the D2DSS is transmitted in compliance with thedownlink reference timing. Since the SA includes various controlinformation for receiving data, even the RRC_Idle UE has to receive theSA. Thus it is specified to transmit the SA in compliance with thedownlink reference timing. In order to avoid ISI or ICI with the SA, theD2DSS is transmitted in the SA resource pool in compliance with thedownlink reference timing which is identical with the SA transmissionreference timing.

In a case where the D2DSS is transmitted in both the SA resource pooland data resource pool as shown in FIG. 7B, the D2DSS transmissionreference timing may be identical with the downlink reference timing oruplink reference timing. For example, in the Mode 1 communication, theSA is transmitted in compliance with the downlink reference timing, andthe data is transmitted in compliance with the uplink reference timing.In order for the RRC_Idle UEs to receive data, the TA information of theTx UE is informed in the SA. If this information is received, theRRC_Idle UEs move up the receiving window to the same extent asindicated by the TA in the SA to receive data. Accordingly, in the Mode1 communication, the D2DSS is transmitted in compliance with thedownlink reference timing in the SA resource pool and in compliance withthe uplink reference timing in the data resource pool. Meanwhile, in theMode 2 communication, the D2DSS is transmitted in compliance with thedownlink reference timing.

In a case where the D2DSS is transmitted in the WAN resource pool asshown in FIG. 7C, the D2DSS is FDM'ed with the cellular data and controlinformation transmitted in uplink. Since the uplink cellular data andcontrol information are transmitted in compliance with the uplinkreference timing (i.e., based on TA), if the D2DSS FDM'ed with them istransmitted in compliance with the downlink reference timing, this maycause ICI or ISI problem.

Accordingly, in a case of the D2DSS transmission in the WAN subframe,the D2DSS is transmitted in compliance with the uplink reference timingas in the WAN. In this case, however, the receiving window configurationmethod may occur at the RRC_Idle UE and thus it is necessary to notifythe RX D2D UEs of the maximum TA value supported in the cell through theSIB. If there is not signaling of the TA value through the SIB, the RxD2D UE moves up the receiving window earlier enough to receive the D2DSSand then perform the reception operation (e.g., it should be specifiedin the standard like 9 SC-FDM symbols). In order to mitigate signalingoverhead and avoid the UE reception operation, the D2DSS may betransmitted in compliance with the downlink referencing timing and, atthis time, it is necessary to set a guard band and guard time largeenough to avoid the ISI and ICI to the WAN.

FIG. 8 is a conceptual diagram for explaining a D2D Tx UE selectionoperation according to an embodiment of the present disclosure.

Referring to FIG. 8, it is assumed that three D2D communication-enabledUEs 801 (e.g., UE-A), 802 (e.g., UE-B), and 803 (e.g., UE-C), asillustrated, are located in a cell 1 601 of a first eNB 811 and one D2Dcommunication-enabled UE 804 (e.g., UE-D) is located in a cell 2 602 ofa second eNB 812. Although one UE 804 is depicted in the cell 2 202 inFIG. 8, there may be two or more D2D communication-enabled UEs in thecell 2 602.

The lower portion of FIG. 8 illustrates TDM'ed LTE communicationresources 831 and 832 and a D2D resource pool 841. It is noted that theD2DSS is transmitted in a first subframe 841 a of the D2D resource pool841. The D2DSS may be transmitted as described with reference to FIGS.7A to 7C.

In the Type 1 discovery, the D2DSS is transmitted at the first subframeof the transmission resource pool for supporting inter-cell D2Doperation. For example, the D2D UEs 801, 802, and 803 of the cell 1 601transmit D2DSS, and the UE 804 or other UEs (not shown) located in thecell 2 602 find the subframe boundary of the cell 1 D2D resource poolusing the D2DSS transmitted by the UEs of the cell 1 601. Afteracquiring the subframe boundary, the D2D UE 804 of the cell 2 602 canreceive the D2D signal from the cell 1 601 using the information on thecell 1 resource pool information transmitted by the cell 2 eNB 812 usingan SIB. Accordingly, in the LTE environment in which the cells are notsynchronized accurately, the D2DSS transmission and reception areinevitable to support inter-cell D2D operation.

At this time, there may be a problem of determining a UE to transmitD2DSS in each cell. For example, since the UE-C 803 of the cell 1 601 isnear the eNB, although it transmits a D2DSS, the UE-D 804 of cell 2 602cannot receive the D2DSS. Accordingly, it may cause unnecessary energyconsumption for all UEs within the cell to transmit D2DSS.

Since it is required to design that the RRC_Idle UE can participate inthe D2D transmission in the Type 1 discovery or Mode 2 communication,the UE may transmit the D2DSS under the triggering condition indicatedin the SIB transmitted by the eNB. For example, the D2D UE measures thereference signal received power (RSRP) form the eNB and, if the measuredRSRP is less than a threshold value X dBm, transmits the D2DSS. At thistime, the value of X dBm may be informed through the SIB transmitted bythe eNB.

In order to avoid the in-band emission problem occurring between theD2DSS and discovery message or WAN data/control information FDM'ed withthe D2DSS, it may be refrained to use the maximum transmit power intransmitting the D2DSS. Depending on whether the D2DSS is transmitted atthe maximum transmit power, the value of X dBm may change. If the D2DSStransmission trigger condition is based on the RSRP acquired bymeasuring the cell-specific reference signal (CRS) transmitted by theserving eNB of the D2D UE, it should be possible to set RSRP valuedifferently depending on the cell radius of the eNB.

In the Type 2B discovery or Mode 1 communication, only the D2D UEs inthe RRC_Connected state can transmit D2D signals. That is, in order forthe D2D UE in the RRC_Idle state to perform D2D transmission, it has totransitions to the RRC_Connected state. Accordingly, in the Type 2Bdiscovery or Mode 1 communication, the eNB may determine the UE totransmit the D2DSS. For example, In the Type 2B discovery (or Mode 1communication), the eNB may command the UE to transmit D2DSS through RRCsignaling.

Meanwhile, since the UE in the RRC_Connected state has acquired the TAinformation from the eNB, the eNB notifies the UE of the threshold valueof the TA through RRC signaling such that the UE determines whether totransmit D2DSS autonomously as in the Type 2 discovery (or Mode 2communication). However, if the UE determines to transmit D2DSS, itreports the determination result to the eNB for use in scheduling at theeNB scheduler.

FIG. 9A is a timing diagram for explaining ISI/ICI occurring when a UE-Atransmits a Type 2B discovery according to UL reference timing aftertransmitting a D2DSS according to DL reference timing, and FIG. 9B is atiming diagram for explaining ISI/ICI occurring when a UE-B transmits aType 2B discovery according to UL reference timing after a UE-Atransmits a D2DSS according to DL reference timing.

In the Type 1 discovery, the D2DSS is transmitted at the first subframeof the transmission resource pool. However, there is no transmissionresource pool in the Type 2B discovery. Accordingly, the D2DSS should betransmitted at the first subframe of the Type 2B discovery receptionresource pool for D2D operation between the cells supporting the Type 2Bdiscovery. At this time, if the D2DSS is transmitted in compliance withthe uplink timing of the serving eNB and if the Type 2B discovery istransmitted in compliance with the uplink timing based on the TA, theICI or ISI problem may occur between the D2DSS transmission frame andthe Type 2B discovery transmission subframe occurring afterward.

Referring to FIG. 9A, a case is illustrated where the UE-A 801, asexemplified in FIG. 8, transmits the D2DSS at the n^(th) subframe incompliance with the downlink reference timing and the Type 2B discoverymessage at the (n+1)^(th) subframe in compliance with the uplinksubframe. In FIGS. 9A and 9B, T_(n) and T_(n+1) denote the transmissiontimings of the cell 1 601 including the eNB 811 in the n^(th) and(n+1)^(th) subframes. These transmission timings are the times in viewof the eNB and thus, in view of the UE, timings may vary depending onthe distance from the first eNB 811. For example, the UE-A 801 mayreceive the n^(th) subframe of the eNB at the timing determined bytaking notice of the propagation delay of T_(A), and the reception timemay become the downlink reference timing. Accordingly, if the UE-A 801transmits the D2DSS according to the downlink reference timing, it maybe transmitted at the time delayed to the same extent as τ_(A). That is,the UE-A 801 transmits the D2DSS at the n^(th) subframe corresponding tothe time delayed to the same extent as τ_(A) from the transmission timeof the eNB.

If the UE-A 801 transmits the Type 2B discovery message at the(n+1)^(th) subframe based on the TA, the end part of the n^(th) subframeand the start part of the (n+1)^(th) subframe may be overlapped. Thismeans ISI occurrence. In order to solve this problem, the followingprincipals have been ratified in a 3GPP RAN1-76BIS meeting.

(1) The last symbol of every D2D subframe is emptied for the purpose ofa Guard period (gap).

(2) In a case where a specific D2D UE transmits a D2D signal at then^(th) subframe without TA information (in compliance with DL timing)and the D2D signal or WAN signal at the (n+1)^(th) subframe based on theTA information (in compliance with UL timing), if the TA value isgreater than 1 symbol, it gives up the D2D signal transmission at then^(th) subframe.

In a case of applying the ratified principal to the D2DSS transmissionwithout modification, if a specific D2D UE transmits a D2DSS at then^(th) subframe and a Type 2B discovery message at the (n+1)^(th)subframe in compliance with the UL timing, it gives up the D2DSStransmission at the n^(th) subframe when TA is longer than 1 symbol.Since the D2DSS transmission is inevitable to supporting inter-cell D2Doperation in the asynchronous network environment, the following methodscan be proposed to solve such a problem.

(1) DL Timing-Based Type 2B Discovery Message Transmission:

In a case of transmitting the Type 2 discovery message in compliancewith the DL timing, it cannot be FDM'ed with the WAN data and controlinformation transmitted in compliance with the UL timing.

(2) UL Timing-Based Type 2B Discovery Message Transmission:

If a D2D UE having TA longer than 1 symbol transmits a D2DSS at then^(th) subframe in compliance with the DL reference timing, the eNBschedules such that the Type 2B discovery is not transmitted at the(n+1)^(th) subframe.

The eNB transmits the maximum TA information of serving and neighboringcells through SIB or PD2DSCH in order for the RRC_Idle UE to receive theType 2B discovery message.

(3) UL Timing-Based D2DSS Transmission in Type 2B Discovery:

It is easy to FDM with WAN data and control information.

There is no need of scheduling restriction of the eNB and operationrestriction of the D2D UE.

The eNB transmit the maximum TA information of the serving andneighboring cells using an SIB in order for the RRC_Idle UE to receivethe D2DSS and Type 2B discovery message.

Referring to FIG. 9B, a case is illustrated where the UE-A 801, asexemplified in FIG. 8, transmits the D2DSS at the n′ subframe incompliance with the DL reference timing and the UE-B transmits the Type2B discovery message at the (n+1)^(th) subframe in compliance with theUL reference timing. That is, the UE-A 801 is scheduled to transmit theD2DSS at the n^(th) subframe, since TA is longer than 1 symbol, but nottransmit the Type 2B discovery message at the (n+1)^(th) subframe.

Meanwhile, the UE-B 802, as exemplified in FIG. 8, is scheduled not totransmit the D2DSS at the n^(th) subframe but transmit the Type 2Bmessage at the (n+1)^(th) subframe. In this case, the ICI/ISI problemmay occur between the D2DSS transmitted at the n^(th) subframe and theType 2B discovery message transmitted at the (n+1)^(th) subframe. Inorder to solve this problem, it can be considered to set a gap (1subframe) large enough. That is, the (n+1)^(th) subframe can be used asthe gap between the n^(th) subframe carrying the D2DSS and the(n+2)^(th) subframe carrying the Type 2B discovery message. Since thegap of 1 subframe causes resource utilization inefficiency, it can beconsidered to use the (n+1)^(th) subframe as dedicated WAN subframe.Accordingly, the bitmap of the Type 2B reception pool always start with10.

FIG. 10 is a diagram for explaining a problem occurring when aneighboring cell Rx UE receives a D2DSS in a Type 2B discovery and thenreceives a Type 2B discovery message transmitted in compliance with ULtiming according to an embodiment of the present disclosure.

Referring to FIG. 10, a UE-D located in a neighboring cell 2 602, asillustrated in FIG. 8, receives a D2DSS transmitted from the cell 1 601,as illustrated in FIG. 8, in compliance with DL timing and then acquiresthe start point of a Type 2B discovery subframe and slot boundary 1011of the neighboring cell.

The UE located in the cell 1 601 transmits the Type 2B discovery messagebased on its TA value in compliance with the UL timing. Since the Rx D2DUE (UE-D) 804, as illustrated in FIG. 8, of the neighboring cell 2 602does not know the TA value, it cannot configure its receiving window.

One method for solving this problem is that the UE which has receivedthe D2DSS at the n^(th) subframe 1000 configures the receiving windowstarting from the start point of the (n+1)^(th) subframe 1001 to receivethe Type 2B discovery message. However, this message may cause discoverysignal reception operation inefficiency. This is because the Rx UE whichdoes not know the TA information of the neighboring cell configure thereceiving window under the assumption of the maximum TA (cell radius of100 km). For example, even though the cell radius of the neighboringcell is less than 1 km, the Rx D2D UE which does not know it configuresthe receiving window under the assumption of the cell radius of 100 km.Receiving windows 1 and 2 extend to a (n+2)^(th) subframe 1002, so as toextend across the (n+1)^(th) subframe 1001 and the (n+2)^(th) subframe1002.

In the present disclosure, the maximum TA value of the serving andneighboring cells are transmitted through SIB or PD2DSCH such that theRx D2D UEs located in the serving and neighboring cells can receive themaximum TA value and adjust the receiving window based on the TA valueindicated in the SIB or PD2DSCH.

FIG. 11 is a block diagram illustrating a configuration of an eNBaccording to an embodiment of the present disclosure.

Referring to FIG. 11, an eNB is illustrated, where the eNB may includean eNB radio communication unit 1101, an eNB data processing unit 1103,an eNB control unit 1105, a network interface 1107, and an eNB memory1109.

The eNB radio communication unit 1101 may be a module for transmittingand receiving data to and from at least one UE located in its coveragethrough a radio channel. The eNB radio communication unit 1101 mayup-convert the data or signals into a radio frequency (RF) band signalsuitable for transmission through an antenna. At this time, the antennamay be comprised of a plurality of antennal elements. Although FIG. 11shows just the functional block configuration, the present disclosure isnot limited thereto. The eNB radio communication unit 1101 maydown-convert the receive RF band signal into a baseband signal. The eNBradio communication unit 1101 may be configured with a component fordetecting received signal strength, which may be provided to the eNBdata processing unit 1103.

The eNB data processing unit 1103 may encode and demodulate the datarequired for communication with UE, such as SIB information which allUEs located in the coverage area of the eNB receive or which dataaddressed to a specific UE, in adaptation to the channel condition withthe UE as described above. The eNB data processing unit 1103 also mayforward the transmit power control information of the encoded andmodulated signal which is provided by the eNB control unit 1105 to theeNB radio communication unit 1101.

Meanwhile, the eNB data processing unit 1103 may demodulate and decodethe data received from the UE. At this time, if the data received fromthe eNB radio communication unit 1101 is analog data, the eNB dataprocessing unit 1103 may include an analog/digital (A/D) converter forconverting the analog data to digital data.

The eNB control unit 1105 may control the overall operation of the eNB,e.g., UE data scheduling operation (operation carried out by thescheduler), UE-specific channel power control operation, and data ratecontrol operation. If it is necessary to communicate with a higher levelnetwork entity, the eNB control unit 1105 may communicate with thehigher level network entity through a network interface 1107. If it isnecessary for the eNB to communicate with a synchronized eNB or aspecific neighboring eNB, the eNB control unit 1105 may communicate withthe neighboring eNB using the network interface 1107 or a separateinterface (not shown). Particularly when it is necessary for the D2D UEsto transmit/receive the D2DSS, the eNB control unit 1105 may control toprovide the condition or command for the D2DSS transmission.

The network interface 1107 may provide an interface according to acommunication protocol shared with a specific higher level networkentity. The network interface 1107 may provide an interface with aneighboring eNB depending on case.

The eNB memory 1109 may include a region for storing specificinformation received from the higher level network entity or datagenerated as a result of the control operation of the eNB controller1105 and another region for storing the program information necessaryfor the eNB control unit 1105 to execute the corresponding programs. TheeNB memory 1109 may store the SIB information and UE capabilityinformation, e.g., whether the UE supports Type 1 communication or Type2B communication, associated with the present disclosure.

FIG. 12 is a block diagram illustrating a configuration of a D2Dcommunication-enabled UE according to an embodiment of the presentdisclosure.

Referring to FIG. 12, a D2D communication-enabled UI is illustrated,where the D2D communication-enabled UE may include a UE radiocommunication unit 1201, a UE data processing unit 1203, a UE controlunit 1205, a display unit 1207, an input unit 1209, and a UE memory1211.

The UE radio communication unit 1201 may be a module for transmittingand receiving data to and from an eNB or a neighboring D2D UE through anRF channel. The UE radio communication unit 1201 may include a cellularcommunication module and a D2D communication module. The UE radiocommunication unit 1201 may include a component for detecting thereceived signal power depending on case, and the received signal powerinformation may be provided to the UE data processing unit 1203.

The UE data processing unit 1203 may perform encoding/decoding andmodulation/demodulation on the data to be transmitted to or receivedfrom an eNB or another D2D UE through a specific channel. The UE dataprocessing unit 1203 may include an A/D converter for converting theanalog data received by the UE radio communication unit 1201 to digitaldata.

The display unit 1207 may provide the user with a graphic interface todisplay the state of the UE and the user input. The display unit 1207may be implemented in various forms of light emission diodes (LED) andliquid crystal display (LCD), but without limitation thereto.

The input unit 1209 is an input module for receiving the inputcorresponding to the user request. The input unit 1209 may be providedin the form of a touch screen or a keypad or both. The input unit 1209may include a voice recognition module for recognizing the user's voice.

The UE memory 1211 may include a region for storing data necessary forcontrol and generating in control and another region for storing thedata which the user requests for storing and information received froman eNB. The UE memory 1211 may include at least one of internal andexternal memories in the form of read only memory (ROM), random accessmemory (RAM), hard disk, or flash memory.

FIG. 13 is a flowchart illustrating a control procedure of an eNB forD2DSS transmission of a UE according to an embodiment of the presentdisclosure.

Referring to FIG. 13, a flowchart is illustrated, where the eNB controlunit 1105, as illustrated in FIG. 11, determines whether the eNB allowsD2D discovery between D2D UEs located in different cells (or inter-celldiscovery) at operation 1300. The D2D discovery may be preset when theoperator installs the eNB or according to the information provided by ahigher level network entity.

If the inter-cell discovery is allowed at operation 1300, the proceduregoes to operation 1302 and, otherwise if the inter-cell discovery is notallowed, the procedure ends.

At operation 1302, i.e., if the inter-cell discovery is allowed, the eNBcontrol unit 1105 determines whether the corresponding cells aresynchronized with each other. That is, the eNB control unit 1105determine whether the cells are synchronized cells. Whether the cellsare synchronized may be determined based on whether the inter-cellsynchronization protocol (e.g., IEEE1588) is running or whether the eNBreceives a synchronization signal from the global positioning system(GPS) (or a separate GPS receiver).

If the corresponding cells are not synchronized, the procedure goes tooperation 1304 and, otherwise if the corresponding cells aresynchronized, the procedure ends. As described above, if thecorresponding cells are not synchronized, it is necessary for the UEs totransmit the D2DSS to discovery them located in the unsynchronized cellseach other.

Afterward, the eNB control unit 1105 determines whether it is allowedfor only the RRC_Connected UE or both the RRC_Connected and RRC_Idle UEsto transmit the D2DSS. The D2DSS Tx UEs recorded in the eNB memory 1109,as illustrated in FIG. 11, may be determined by the operator. It may beconfigured dynamically when the operator installs the eNB or based onthe information provided by a higher level network entity.

If only the RRC_Connected UE is allowed to transmit the D2DSS atoperation 1304, the procedure goes to operation 1308 and, otherwise ifboth the RRC_Connected and RRC_Idle UEs are allowed to transmit theD2DSS, the procedure goes to operation 1306. At operation 1306, the eNBcontrol unit 1105 controls the eNB data processing unit 1103, asillustrated in FIG. 11, and the eNB radio communication unit 1101, asillustrated in FIG. 11, to transmit a command such that both theRRC_Connected and RRC_Idle UEs transmit the D2DSS. Since this commandtransmission scheme has been described above, detailed descriptionthereof is omitted herein.

Meanwhile at operation 1308, i.e., if both the RRC_Connected andRRC_Idle UEs are allowed to transmit the D2DSS, the eNB control unit1105 controls the eNB data processing unit 1103 and the eNB radiocommunication unit 1101 such that the eNB broadcast the RSRP thresholdfor D2DSS transmission condition.

FIG. 14 is a flowchart illustrating a D2DSS transmission procedure of aUE according to an embodiment of the present disclosure.

Referring to FIG. 14, a flowchart is illustrated, where the UE controlunit 1205, as illustrated in FIG. 12, may know whether it is operatingin the RRC_Connected state or RRC_Idle state. In a case of the UE in theRRC_Connected state, the UE control unit 1205 may receive the D2DSStransmission command from the eNB through dedicated RRC signaling bymeans of the UE radio communication unit 1201, as illustrated in FIG.12, and the UE data processing unit 1203, as illustrated in FIG. 12. Thecontrol flow of FIG. 14 starts with the step of determining whether theD2DSS transmission command is received.

If it is determined that a D2DSS transmission command is received atoperation 1400, the UE control unit 1205 controls the UE radiocommunication unit 1201 and the UE data processing unit 1203 to transmita D2DSS at operation 1408.

If it is determined that no D2DSS transmission command is received atoperation 1400, the UE determines whether an RSRP threshold is receivedfrom the eNB at operation 1402. As described above, the RSRP thresholdmay be a condition for determining whether to transmit the D2DSS. Thatis, the UE which is in a non-RRC_Connected state, i.e., RRC_Idle state,or which is in the RRC_Connected state but have not received the D2DSStransmission command determines whether the RSRP threshold for use indetermining D2DSS transmission is received through a SIB at operation1402.

If it is determined that the RSRP threshold is received through a SIB atoperation 1402, the UE control unit 1205 measures RSRP from the eNB atoperation 1404. At this time, the RSRP may be measured newly for D2DSStransmission or may be the value measured already in the cellular modefor cellular communication of the D2D UE, e.g., uplink transmit powercontrol. That is, the RSRP may be a value measured after operation 1402or measured already for other purposes before operation 1402. If it isdetermined that the RSRP threshold value is not received at operation1402, the UE control unit 1205 controls the UE radio communication unit1201 and the UE data processing unit 1203 to transmit a D2DSS atoperation 1408.

Next, the UE control unit 1205 compares the measured RSRP value with theRSRP threshold received through the SIB at operation 1406. If the RSRPmeasurement value is less than the RSRP threshold, the procedure goes tooperation 1408 and, otherwise if the RSRP measurement value is equal toor greater than the RSRP threshold, the procedure ends without D2DSStransmission.

FIG. 15 is a flowchart illustrating a signal reception procedure of a UEfor receiving a D2D discovery, a control signal or data from another UElocated in a neighboring cell according to an embodiment of the presentdisclosure.

Referring to FIG. 15, a flowchart is illustrated, where the UE controlunit 1205, as illustrated in FIG. 12, receives an SIB from a servingcell, i.e., eNB (or cell) to which it is connected at operation 1500.The UE control unit 1205 may acquire the information on the resource forreceiving the discovery signal from the neighboring cell or controlinformation (SA)/data information from the SIB. At this time, the SIBmay include an identifier (ID) of the D2DSS transmitted from theneighboring cell as well as the D2D resource information of theneighboring cell.

At operation 1502, the UE control unit 1205 determines whether thereceived SIB includes the neighboring cell resource information atoperation 1500. If the SIB includes the neighboring cell resourceinformation at operation 1502, the UE control unit 1205 may receive theD2DSS based on this information at operation 1504. If the D2DSS isreceived, this means that the aforementioned condition is fulfilled.That is, the UE is in the range capable of performing D2D communicationwith a UE located in a neighboring cell. If the SIB does not include theneighboring cell resource information at operation 1502, the UE controlunit 1205 ends the procedure.

Afterward, if the D2DSS is received at operation 1504, the UE controlunit 1205 may acquire synchronization using the D2DSS transmitted by theD2D UE located in the neighboring cell. If the UE acquiressynchronization with the D2D UE of the neighboring cell in this way, theUE control unit 1205 determines the start point of the subframe based onthe D2DSS and receives at least one of the discovery signal or controlor data information transmitted by the D2D UE of the neighboring cell insubsequence to the D2DSS subframe at operation 1506.

In an asynchronous network, it is necessary for the UEs to acquiresubframe boundary timing of the neighboring cell by transmitting andreceiving D2DSS to support D2D operation between D2D UEs located indifferent cells. The present disclosure is advantageous in that the D2DRx UE to operates efficiently by specifying the operation of the D2D RxUE.

Various aspects of the present disclosure can also be embodied ascomputer readable code on a non-transitory computer readable recordingmedium. A non-transitory computer readable recording medium is any datastorage device that can store data which can be thereafter read by acomputer system. Examples of the non-transitory computer readablerecording medium include Read-Only Memory (ROM), Random-Access Memory(RAM), CD-ROMs, magnetic tapes, floppy disks, and optical data storagedevices. The non-transitory computer readable recording medium can alsobe distributed over network coupled computer systems so that thecomputer readable code is stored and executed in a distributed fashion.Also, functional programs, code, and code segments for accomplishing thepresent disclosure can be easily construed by programmers skilled in theart to which the present disclosure pertains.

At this point it should be noted that various embodiments of the presentdisclosure as described above typically involve the processing of inputdata and the generation of output data to some extent. This input dataprocessing and output data generation may be implemented in hardware orsoftware in combination with hardware. For example, specific electroniccomponents may be employed in a mobile device or similar or relatedcircuitry for implementing the functions associated with the variousembodiments of the present disclosure as described above. Alternatively,one or more processors operating in accordance with stored instructionsmay implement the functions associated with the various embodiments ofthe present disclosure as described above. If such is the case, it iswithin the scope of the present disclosure that such instructions may bestored on one or more non-transitory processor readable mediums.Examples of the processor readable mediums include Read-Only Memory(ROM), Random-Access Memory (RAM), CD-ROMs, magnetic tapes, floppydisks, and optical data storage devices. The processor readable mediumscan also be distributed over network coupled computer systems so thatthe instructions are stored and executed in a distributed fashion. Also,functional computer programs, instructions, and instruction segments foraccomplishing the present disclosure can be easily construed byprogrammers skilled in the art to which the present disclosure pertains.

While the present disclosure has been shown and described with referenceto various embodiments thereof, it will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present disclosure asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A synchronization method for device-to-device(D2D) communication with a terminal, the method comprising: receiving,from a base station, configuration information associated with atransmission of a D2D synchronization signal (D2DSS); receiving atransmission power parameter of the D2DSS from the base station;transmitting the D2DSS based on the configuration information and thetransmission power parameter of the D2DSS; based on the configurationinformation indicating that D2DSS transmission is not commanded,receiving, from the base station, a threshold value for determiningwhether to transmit the D2DSS; measuring, from the base station, areference signal received power (RSRP) value; and in response to themeasured RSRP value being less than the threshold value, transmittingthe D2DSS.
 2. The method of claim 1, wherein the configurationinformation is received when the terminal is in a radio resource control(RRC) connected state.
 3. The method of claim 1, wherein theconfiguration information includes a D2DSS transmission command fordirecting D2DSS transmission.
 4. The method of claim 3, wherein,if-based on the configuration information indicating that D2DSStransmission is commanded, the configuration information furtherincludes a power parameter of the D2DSS.
 5. The method of claim 1,wherein the transmission power parameter of the D2DSS is set as amaximum power of the terminal.
 6. The method of claim 1, wherein thethreshold value comprises a D2DSS threshold value that is received via asystem information block (SIB) from the base station.
 7. The method ofclaim 1, wherein the transmission power parameter of the D2DSS istransmitted via a radio resource control (RRC) signaling.
 8. Anapparatus for device-to-device (D2D) communication in a terminal, theapparatus comprising: a transceiver configured to transmit and receivedata to and from a base station or a neighboring D2D terminal; and aprocessor configured to: control the transceiver to receiveconfiguration information associated with a transmission of a D2Dsynchronization signal (D2DSS) from the base station, control thetransceiver to receive a transmission power parameter of the D2DSS fromthe base station, control the transceiver to transmit the D2DSS based onthe configuration information and the transmission power parameter,based on the configuration information indicating that D2DSStransmission is not commanded, control the transceiver to receive, fromthe base station, a threshold value for determining whether to transmitthe D2DSS, measure a reference signal received power (RSRP) value, andin response to the measured RSRP value being less than the thresholdvalue, transmit the D2DSS.
 9. The apparatus of claim 8, wherein theprocessor is further configured to, when the terminal is in a radioresource control (RRC) connected state, control the transceiver toreceive the configuration information from the base station.
 10. Theapparatus of claim 8, wherein the configuration information includes aD2DSS transmission command for directing D2DSS transmission.
 11. Theapparatus of claim 10, wherein, if based on the configurationinformation indicating that D2DSS transmission is commanded, theconfiguration information further includes a power parameter of theD2DSS.
 12. The apparatus of claim 8, wherein the transmission powerparameter of the D2DSS indicates a maximum transmission power of theterminal, and wherein the processor is further configured to set a D2DSStransmission power to the maximum transmission power of the terminal.13. The apparatus of claim 8, wherein the threshold value comprises aD2DSS threshold value that is received via a system information block(SIB) from the base station.